Water overflows into a pipe through a bulkhead and into your sump. Generally into a sock to filter solids. In a basic sump there is only one chamber. So from the tank into the sock and into the sump, you likely will have a protein skimmer in the sump. Water goes into the protein skimmer and then out of the skimmer and back into the sump. Your return pump is also in your sump. This pump will push water back up through a 2nd bulkhead back into the tank.
Once you see it you'll understand, it's very basic and easy.