A modifier is a number, usually an integer, that is added to or subtracted from the total on a dice roll. They are usually written (e.g. in dice notation) as +x or -x, where x is the value of the modifier and the + or - indicate whether the value should be added to or subtracted from the unmodified total.
Modifiers are common in resolution mechanics, because they can change the probabilities of success or failure (as part of the execution step of resolution, particularly when using roll-over or roll-under mechanics) or alter the effect. For example, in Dungeons & Dragons 5th edition, separate (albeit connected) modifiers apply to attack rolls (determining success or failure) and the subsequent damage rolls (determining the effect of the attack).
Modifiers may be based on character traits (especially statistics), the difficulty of the task, or other factors in the game world that affect the situation. In some games, they can be influenced by spending player resources like metacurrency (e.g. spending fate points to invoke an aspect in Fate Core gives a +2 to a roll).