Definition of loose
What is meant by loose is "a loose range of hands played". If a player is said to play loose, he is opening a loose fan hand. The player who plays loose finds reasons to play more hands than other players. This fact is inferred when you see his stats in his tracker or on his HUD. For example, on a full ring table, a classic player will play between 8% and 12% of his UTG hands. A player playing loose would play up to 35% of his UTG hands (sometimes more).
Loose preflop, loose post-flop
Usually, players who play loose-preflop (you could also say, players who play too many hands) will also tend to play too many hands after the flop. They will often call too many with pairs that have bad kickers, they will tend to over-evaluate their hands, they will tend to call big bets without the necessary odds. This is not the profile of the player you want to bluff. These players will not be known for their "hero-fold". Against this style of player, it is best to adopt a TAG style and bet big when you have a strong hand.