An active player can mean one of two things depending on the context of the phrase:
A player at a table is considered active at a table between hands, or in a larger general sense, if they are not sitting out at the table; that is, if they have been or will be dealt a hand at the start of a poker hand. The number of active players at a table is a key factor in evaluating the strength of your starting hand; with fewer players at the table, relative hand values go up since there are fewer possible opponents.
A player in a hand is considered active if they have not yet folded their hand and they are not all-in, so they still have chips with which they could bet in a betting round. The number of active players in a hand is a key factor in calculating implied odds.