Sounds like you have the idea. The box usually has teeth to just take in the surface of the water and keep critters out. The surface is where the oils and proteins collect, so that water is drained to the sump to be ran through your skimmer and maybe some rock/algae ect. to clean it further. The clean water is then pumped back into your main tank and this overflows the box, making more water run back to your sump to be cleaned.