A "rip current" is a seaward-moving current. Try to imagine a sandy underwater river suddenly forming and occurring on the bottom of the lake.
"Rips" are created when accumulating waves on shore use gravity to get back into open sea. Something gives - the sandbars on the lake floor and everything in its path is catapulted out into the lake.
Rip currents are sometimes, mistakenly called "rip tides" or "undertows." Although this terminology attempts to refer to similar phenomenon, it is a bit misleading, since rip currents are not directly associated with tides and they do not "pull" people under. Rip currents pull victims away from the beach, farther out into the lake.