Consistency in fantasy football describes how predictable a player's scoring is. A consistent player puts up similar point totals most weeks, so you know roughly what you're getting when you start him. An inconsistent player might explode for a huge game one week and then disappear the next, which makes him risky to rely on.
Consistency matters because fantasy seasons are won and lost week to week. A player who scores 12 points every week can be safer to start than a player who alternates between 30 points and 3, even if the two average the same. Managers often check a player's week-by-week game log to spot boom-or-bust patterns before deciding who to start or trade for.
Consistent players are especially valuable in season-long leagues during the playoffs, when one bad week can end your season. Boom-or-bust players, on the other hand, are often more useful in daily or best-ball formats, where a single big game can pay off without the risk of a bad start hurting you.