The catcher is the battery mate of the pitcher. If you missed Lesson #1 about the Pitcher, stop reading this post immediately and click HERE. His primary job is to catch the ball from the pitcher. He also has a lot of say in what type of pitch the pitcher throws. The catcher is just as responsible for knowing the batter as well as the pitcher. He's gives the pitcher a sign to tell him whether to throw a fastball, a curveball, a splitter, or whatever type of pitch they think will best fool the guy at the plate.
Because of where the catcher is positioned, he can see the whole field so he leads the defense. He directs the other players and typically will signal to other infielders (who'll you'll learn about next week) if a special play is needed because of the base running situation, or the tendencies of the guy currently up to bat. A catcher is one of those guys who can be on a team purely for his defense, even if his offensive skills are sub par. But ideally... it's good if the catcher can hit too!
The catcher usually has to be the toughest guy on the field. Not only is he constantly peppered with errant foul balls, but he also can be involved in home plate collisions. It's the one base where the runners can throw themselves at the catcher to try and dislodge the ball—at least it was until Major League Baseball outlawed these collisions for the 2014 season. Catchers also find themselves defending the pitchers quite often. If the pitcher has just hit the batter with a pitch and that batter takes offense and charges after the pitcher, it's the catcher's job to tackle that a-hole before he gets to the mound.
If you're still confused about the catcher and what he does and how important he is to calling the game and how dangerous it can be for the pitcher to not follow his instructions, please watch thes highly educational video clips. The handsome guy wearing that funny blue chest protector—he's the catcher.
I recommend watching the entire
movie instructional video... it's very helpful for understanding the relationship between pitchers and catchers.