From BR Bullpen
"All baseball fans can be divided into two groups: those who come to batting practice and the others. Only those in the first category have much chance of amounting to anything." - Thomas Boswell
Batting practice is the period during which players take a few swings with the bat to get ready for an upcoming game. Batting practice usually takes place one to two hours before a scheduled game, on the field of play itself. A batting cage is rolled onto the field to prevent foul balls from flying in all directions, as the ballpark is often open for spectators during the period. Batting practice is a fruitful time for spectators to collect baseballs hit into the stands, as home runs or long foul balls.
Batting practice takes place with coaches or dedicated batting practice pitchers, standing behing a partial screen, throwing slow fastballs at the plate. Batters will work on getting their timing ready for the game, and will usually attempt to do a few things like hit to the opposite field or lay down a bunt. Meanwhile, other players will stand in the outfield, shagging fly balls. Each batter has a pre-set number of swings, and batting practice etiquette is very strict about not exceeding this number and not jumping one's turn, as there is a maximum time allotted for the team and every player needs to have a chance to take a few cuts.
In certain circumstances, special events taking place before the game will cancel or move the time allotted for batting practice. Players will often moan when that is the case, as for many, it is a key part of their pre-game routine.