In baseball, a backstop is the wall or fence behind home plate that keeps the ball on the playing field. If the batter hits the ball into the backstop, it's a foul ball.
When a softball player is up at bat, the backstop is behind her. Part of the backstop's purpose is to keep players and spectators from getting hurt by foul balls or bats that players toss after hitting the ball. You can also use the word backstop to describe a thing or a person that stands behind something, offering reinforcement or support. The earliest use of the word, in 1819, is from cricket.