General layman’s term used to describe coastal currents which may “suck” swimmers underwater. An undertow is a short-lived, sub-surface surge of water associated with wave action. It can drag you down, but it’s not truly treacherous because you usually won’t be held under for long. Just relax and hold your breath, and you’ll pop to the surface. Generally what people are referring to when the say "undertow" is backwash from large breaking waves or seaward-flowing rip currents.