Loose
A loose player is a player who plays a lot of hands, including weak ones. He enters many pots and is less selective than the average player. The opposite style is tight.
The simple explanation
Loose players play many hands. They call a lot and see many flops. Against them, you should value bet your strong hands, because they call with weaker hands, and bluff less.
A closer look
Looseness describes how many hands a player plays, not how aggressive he is. A loose-aggressive (LAG) player plays many hands and bets and raises often. A loose-passive player plays many hands, but calls rather than raises, and is called a calling station. The right strategy depends on the type. A loose player may be profitable if skilled but is often a weak player who can be exploited by tighter, patient play.
A practical example
A player calls a raise with 9♣ 4♦ and later plays J♣ 3♥ from early position. He is playing loosely.
Full explanation
A loose player is a player who plays a lot of hands, including weak ones. He enters many pots and is less selective than the average player. The opposite style is tight.
A loose player plays more hands than average.
Loose is about the number of hands, not about aggression.
Loose and aggressive players are called LAGs.
Against loose players, value bet more.