You have one Pokemon in the middle of the table, which is considered your "active" Pokemon. this is the Pokemon you can attack with. All other Pokemon "behind" the active Pokemon are considered "bench" Pokemon. You may not attack with bench Pokemon.
Sometimes a card will talk about "choose an Active" or "one of your opponent's Active Pokemon".
That wording is for 2-on-2 play when each player has two Actives and only up to 4 on the Bench