Statbook: Frequently Asked Questions

Catchers interference can happen when a batter swings and the bat strikes the catchers glove or helmet. It does not count as an official at-bat but does count as an error on the catcher. Here is the full description from Wikipedia:

Catcher's interference is called when the catcher physically hinders the batter's opportunity to legally swing at a pitched ball. This occurs most often when a catcher squats too close to home plate, so that the batter's bat touches the catcher's glove as the batter swings. This is most likely to happen on attempted steals where the catcher is anxious to catch the ball as soon as possible and may move his entire body or glove forward a bit.