A player who is defined as an aggro type is an aggressive player in general. To be more precise, this is a player who will tend to favor aggressive poker actions (betting, raising, check-raises, all in, bluffs), rather than passive actions (folding, folding, giving up easily). Aggro players are probably the most formidable opponents since they will never give you an easy time. They are the ones who can check-raise very wide on the flop, who can bluff often. Against this type of player, you can expect a lot of variance.
Aggro player vs. passive
All good poker players agree that winning poker is aggressive poker. You don't win pots by folding every hand. And you win bigger ones by being aggressive. In poker, good hands are rare. Simply waiting for your hand won't be enough to win in the long run. By being aggro, you win uncontested pots. For example, in a hand, if two players do not have good cards, the more aggressive player (aggro) will win the pot.