From BR Bullpen
Catcher's interference is a specific type of interference that occurs when the catcher makes contact with the batter (or his bat) during a pitch, or otherwise hinders or impedes a batter's ability to hit a pitched ball. In order for catcher's interference to be enforced, the batter must have been in a legal batting position with both feet within the batter's box.
Catcher's interference is considered a delayed dead ball situation, much like a balk. If a play follows the interference, the umpire will allow the play to continue, as the offended team's manager can choose to decline the interference and accept the result of the play, in which case the play stands without reference to the interference. If the interference is accepted, the batter is awarded first base without liability to be put out, and baserunners may advance only if forced. The catcher is charged with an error however the batter is not considered to have reached on an error, and is not charged with a time at bat. While the batter is charged with a plate appearance (according to rule 10.22a), for statistical purposes, reaching on catcher's interference does not affect a player's on-base percentage. A notation should be made in the boxscore that batter x was awarded first base because of catcher's interference.