He must be referring to riptides. In some spots the water hits the beach as waves. In others nearby, the water gets pulled back into the ocean, and those are the spots you need to avoid.
Then depending on the ebb and flow of the twice-daily tides, the riptides are stronger or weaker.
There are ways to see where the riptides are, yet many people from my own coastal town are oblivious to these dangers. Inland/landlocked tourists are even more oblivious and vulnerable.