From Baseball-reference.com, where Charlie Brown has an entry:
"He is perhaps best known for his stubborn determination to win a baseball game, and for refusing to give in when all seems lost (he can be seen standing on the pitcher's mound alone during a torrential downpour, refusing to let a little rain ruin his beloved baseball game).
One recurring theme in the strip is Charlie Brown's Little League baseball team. Charlie Brown is the manager of the team and, usually, its pitcher, with the other characters of the strip comprising the rest of the team. Charlie Brown is a terrible pitcher, often giving up tremendous hits which either knock him off the mound or leave him with only his shorts on.
You may not remember it or maybe missed it, but this was a running motif, just like the football-pull and kite-eating tree and Joe Cool and a dozen other things.