Success is the accomplishment, to some extent, of a character's or player's intentions at a moment of conflict or uncertainty that has required resolution. It is the opposite of failure, in which the intentions were not accomplished, and is generally determined during the execution step of a resolution mechanic (i.e. after the character's initiation has been established, but before the effect of the success or failure is worked out). For example, a weapon attack is successful if the attacker hits their enemy.
While the terms success and failure imply a binary outcome to a resolution's execution, most resolution mechanics are more complicated than this, either by using a distinct effect stage (e.g. separate damage rolls after a hit) or by the mechanic having multiple possible outputs representing degrees of success (e.g. allowing for critical hits or partial success or opportunities that are otherwise independent of success or failure).
Success indicates that a character (or player) has accomplished what they intended, but it does not always follow that they have accomplished what they wanted. This is one distinction between a task resolution system (in which the character may succeed at an action without gaining the benefits they expected) and a conflict resolution system (in which the character may achieve the benefits, but will not necessarily have performed the action they expected). The extent to which successful actions and successfully gaining benefits are equivalent overlap may be determined by the game rules or by the GM.