One of The Six Stats, abbreviated as INT.
This is the stat that determines how much a character can learn and remember, along with their ability to reason. Intelligence is the go-to measuring stick for completing puzzles, understanding complicated incantations and constructing marvels of science and technology, depending on the setting.
Typically a Dump Stat for testosterone-powered bruisers and other macho men and women, although Genius Bruisers overcome this simple dichotomy. High Intelligence is almost mandatory for a villain to concoct their master plan, and it is one of two defining stats for the Magnificent Bastard. For this reason, heroes often dump this stat, but don't be fooled into thinking that this is a general rule - the Guile Hero is an archetype of their own.
Here's a guide for examples:
12-15: At this level, Intelligence appears as little more than an academic gift, resulting in high scores on IQ tests, but nothing record-breaking.
16-19: The character is probably the token smart guy of their group, unless they operate alone. Technical expertise, whether it be about the workings of magic or how to fix a computer, comes easy - this talent usually prompts the character to try and challenge the limits of their mind.
20-23: Learning a language before breakfast is almost a normal day. Examinations prompt nothing more than a yawn. Encyclopedic knowledge is a fair descriptor, though possibly inadequate.
24+: This character literally knows more than you'll ever forget. They understand pi to six billion digits, dabble in research that would leave savants babbling in sheer incomprehension and you'd better believe they know at least as many ways to take a person down as there are grains of sand on a beach, magically, scientifically or otherwise.