The cage is Batting Cages used to keep the baseballs within a certain amplitude so that they're easy to pick up and are not lost. Batting cages are often found indoors, or where blank is limited.
The defense attempts to get the batter out. The pitcher's main role in this is to throw the ball in such a kick that he or she either strikes out or cannot hit it cleanly so that the defense can get him or her out.