The wicketkeeper stands behind the striker. He has a big leather glove, which he uses to catch balls the striker misses.
The wicketkeeper tries to bring down the bails if the striker is over the batting crease.
The fielding players can also try to throw the ball to the wicketkeeper, if the batsman is over the batting crease, so the wicketkeeper can knock down the bails.
The wicketkeeper will be right behind the wicket if the bowler is a spinner, or further away if the bowler is a fast-bowler.