What's a catcher's balk?
A catcher's balk happens when a pitch is thrown and the catcher is not completely within the catcher's box which is supposed to be outlined in chalk before the game starts.
There are a couple of problems with the rule:
1. The chalk is often worn out by the third inning of a ball game making the call harder to make - and that's assuming that,
2. They bother to actually draw the catcher's box which is not a frequent practice nowadays.
It's kind of a misnomer in that the "balk" and the advancing baserunners including any runs scored are charged to the pitcher.
Essentially, it's an extremely rare call (I think the last one was in the early 2000's and even that was the first time in many, many years) and if I was a pitcher and the umpire called a "catcher's balk" I'd be tempted to punch the catcher in the face.