There are many things in life that can cause harm to oneself, and this harm can range from something like a simple scratch or stubbed toe to death, from a moment of annoyance or anger to the destruction of a valued relationship or someone undergoing emotional torment. The philosophy of "harm reduction," most generally discussed in regard to behaviors generally considered extremely self-destructive such as drug use and Self-Harm, but which can be applicable in many other contexts, is a philosophy of being mindful of the harm one's behavior does to oneself, and planning, whether with a therapist or friend or family member or even oneself, ways to reduce the harm of a given behavior and to accept oneself, even if the behavior is never thoroughly abstained from or eradicated.
Principles of harm reduction philosophy
Total, complete abstinence from the behavior is impossible or nearly impossible for the person to achieve in their present circumstances, not something the person desires enough to achieve, and at worst actively sets the person to fail, with the result of the failure to abstain being possibly more harmful than the continued behavior.
As an example, sex is one of the strongest drives of most humans who are not Asexual or demisexual, and even some who fall into the asexual or demisexual groups still feel a drive for physical sexual release on occasion. This makes most demands for sexual abstinence upon human beings almost doomed to fail aside from the most strong-willed and the most disinterested anyway (who make for most of the "success" of abstinence). Abstinence-only approaches to sex, therefore, run a very high risk of failure, and when "don't do this and if you do you'll go to hell," for example, is the only education one recieves on sex, and they have it anyway, they are far more likely to do so in an unsafe manner and get pregnant or get an STD.
This principle of harm reduction is not judging those who do succeed at abstinence or who even make it their life's goal to abstain from an activity they see as specifically harmful to themselves - it is simply offering an alternative to abstinence that, while not as "safe" as abstinence is, is safer than entirely going Off the Wagon or doing something with inherent danger in the most dangerous way possible.
All human behavior carries some risk, some behaviors are riskier than others to health or life. Not all risk from any given behavior can be eliminated without total abstinence, but with almost any dangerous behavior, there are ways to limit the risks, sometimes in ways that bring the risk level almost down to that of abstinence, and that at the very least reduce the chance of death somewhat.
Using the sex example above, unprotected vaginal intercourse carries a high risk of pregnancy for the receptive partner. Unprotected anal intercourse with an HIV-positive partner carries an even larger risk of AIDS for the receptive partner. In both cases, the insertive partner wearing a condom cuts the risk down to less than 1 percent in the pregnancy example, and less than 2 percent (anal sex) or 0.08% (vaginal sex) in the active HIV example in any one sex act. So there is still risk (which is why the But We Used a Condom trope exists) but the risk goes from high to small.
To use another example, drug use, a heroin addict using street heroin, a dirty needle, and without naloxone available faces a plethora of risks: overdose, instability of supply, and getting anything from cellulitis to HIV. A heroin addict using medical heroin with standardized amounts, clean needles, and with naloxone available to reverse an overdose is still engaging in a very risky behavior with severe health consequences - but his or her risk of immediate death and of getting cellulitis or HIV (and spreading it to others) has been reduced.
In a Self-Harm example, someone who regularly cuts their wrists with a razor without cleaning it or knowing where their veins are risks accidental suicide, severe infection, severe blood loss if they do hit a vein, nerve damage, open scarring, and similar risks. However, if someone avoids the wrist and major arteries, veins, and tendons on knowing where they are and cuts on the fleshy parts of the arm instead without deep pressure and with a sterile lancet, or better yet, settles for snapping a rubber band against the flesh or scratching the flesh with something that doesn't break the skin, almost all of these risks are highly reduced (and with the rubber band or scratching that doesn't break the skin example, are cut to almost nil). The Self-Harm is still happening but is no more dangerous to the person than any other more "innocuous" behavior.
Providing education (and ideally resources and supplies) on how to engage in a risky behavior in a way that causes the least harm possible (or at least very much lowers risks such as immediate death, communicable disease, and harm toward other people) is essential and ethical, and it does not encourage those who would not participate in the behavior to begin with to take it up on a large scale.
Harm reduction philosophy expressly rejects the idea that education and providing alternatives other than abstinence is somehow unethical or allowing people to "wallow" in their socially disapproved or self-destructive behaviors (for example, drug users who practice harm reduction aren't necessarily doomed to a "life on drugs," some may very well move to abstinence or at least partial abstinence, but will stay alive and healthier in the meantime before they do, and even if they do have a "life on drugs" it is far less likely to harm themselves or especially others) and it also rejects the idea that risk reduction education encourages people to engage in self-harming behavior, at least when the messages are directed properly.