Poker range (or preflop chart) is a set of hole cards you (or your opponent) can have in a particular game situation. All players have a 100% range when a hand is just started since everyone can be dealt absolutely any hand.
With every action a player takes, their preflop range gets narrower and narrower, since the player is able to perform each action only with certain hands. For example, this is a typical range of 3Bet or Call from Small Blind against Button (total defense is around 16% of hands).
Check out our more detailed guide to poker ranges.