"Pinch hitter" is a baseball term for a substitute hitter. He replaces the normal man in the lineup. A pinch hitter is normally used, when you are only a few points behind in a late inning and you hope that you can win the game with this hitter. Example you are the away team, and your team is 1 run behind and you play in a league without a designated hitter. After the bottom 8th inning you replace your pitcher (starter or setup man) with a pinch hitter. Hopefully he helps you to achieve a lead. So after the top 9th inning you replace him again with your stopper, who hopefully saves the lead.

But you can not only replace pitcher with pinch hitters, but every other player, too. But you shouldn't have to defend any more, or you should have a good defender on the bench (ex.: someone who hits bad). This is important, as most pinch hitters are only good at hitting. Because of this pinch hitters are very similar to normal designated hitters. They are often some of the older guys, who can still hit, but are not very good at playing defense anymore (maybe they've never been, but they were such good hitters, that it wasn't important).