A rule introduced into Major League Baseball in 2014 that prevents the catcher from 'blocking' the runner from getting to home plate. The rule was added to help reduce physical impact and risk of injury to the base runner and catcher.
Fielders often block bases, such as 2nd base. Because home plate is the 'final destination' the base runner just needs to get to the plate - as opposed to 2nd or 3rd base where the runner must maintain contact with the base (they are required to maintain some control in their running style). The result at home plate was all to often base runners trying to run through the catcher to either inflict injury or jar the ball lose in an effort to be safe and score.
Pitchers are exempt from the blocking the plate rule.