It pretty much does exactly what the words mean.
It receives data from the main router and then retransmits a copy back out again.
These are mostly used when you have poor or no coverage in a remote location. If you place a repeater 1/2 between the router and the remote location many times you can get a usable signal in the remote location. There are many downsides to doing this so it is not a recommended way to accomplish this but many times people have no other options.