RangeThe range of a player is all the possible cards with which he would play in a given situation. If a player in Texas Hold'em raises only AA, KK, QQ, and AK, for instance, then his range is: (AA,KK,QQ,AK), also denoted (QQ+, AK).
Meaning in the Game
In the game, this translates to a question: with which hands would this player act as he has in this situation? It might not matter, for instance, whether the player has 55,66,77 or 88. If he plays all these hands the same, then it is enough to know that he has a middle pocket pair.
The equity of one's own hand comes from the chances it has to win against the range of the opposition. An opponent might hold a stronger or weaker hand from his range, but as long as the range is well-estimated and the play against his range is correct, then it doesn't matter which particular hand from his range he has.
Kinds of Ranges
The range of a player always depends on the situation, the player, the opposing action, and the player's actions.
Hands with which a player would raise a bet.
Hands with which a player would raise first-in.
Hands with which a player would go all-in.