A plot point is any event, appearance of characters or objects, or any kind of piece of information which is integral to driving the plot towards its conclusion.
Often a plot point will be introduced at the beginning of a story, passed off by the characters as something obscure or insignificant, only to later be revealed to them as very important to, or indicative of, events that take place later. Writers can be very subtle in weaving their plot points into their stories, but if you are looking for them, The Law of Conservation of Detail will usually give them away, since the author must make such plot points — whatever form they may take — memorable enough that it doesn't feel like an Ass Pull when it comes time to resolve them. In contrast to Foreshadowing, which may be no more than a hint at things to come added for artistic fluff, plot points are crucial to following the plot. If you miss one, you'll likely be scratching your head later as events play out with no obvious impetus.
Please don't list examples of this Omnipresent Trope and don't include it as a trope on a work's page!