Loose line sportsbooks are those that shade their bets. If a football match has one team legitimately calculated as a 6 point favorite and the operator can see lot of money going on the game, a Loose Sportsbook will move the line maybe to make it a 7 point favorite.

Another reason a sportsbook could be considered loose is if they just have different numbers. With so many betting advice websites, many people have a good idea what the “true” number is for a game. 

A sportsbook that moves its number based solely on the action and not just following the crowd, will also have different lines on occasion and can thus also be considered loose.