Most commonly, aggression in poker alludes to the manner in which a player is raising or betting instead of just calling, limping, or checking. The more raising and betting the player does compared to just calling and checking, the more aggressive he is considered to be. Most regulars and poker sharks play fairly aggressive poker strategies.
Additionally, this term can refer to a particular action, such as a specific raise or bet. Strategic aggression can be very effective since players who know when to play aggressively or stick to GTO are generally more successful than those who play passively.
For example, let’s say that you have a strong top pair or better hand after the flop, and your opponent bets. In this scenario, the aggressive line would be to raise and go for a value bet instead of playing it passively and just calling.