At some point the need may arise for a coach to visit his pitcher out on the mound to calm him down or discuss strategy about how to pitch to the upcoming batters.
The manager (or any coach) is permitted to visit his pitcher one time per inning. The second time he visits that pitcher, he must take him out of the game. If he brings in a second pitcher, the same rule applies, one visit is good, but he must take him out on the second visit.
Usually the pitching coach will come out during the first visit, but the manager will be the guy to take the ball from his pitcher and pass it on to the next guy.
This is unrelated to catcher visits to the mound, which are unlimited.