What does the title of the text mean?

Dreaming brain rhythms lock in memories

It is the clearest evidence to date that REM sleep is critical for memory. By switching off certain brain cells, the researchers silenced a particular, rhythmic type of brain function - without waking the mice. If they did this during REM sleep, the mice failed subsequent memory tests. The research is reported in the journal Science.

REM sleep is the phase during which, at least in humans, dreams take place - but the question of whether it is important for settling new memories has been difficult to answer. Recent studies have tended to focus on deep, non-REM sleep instead, during which brain cells fire in various patterns that reflect memory consolidation and "re-play" of the day's experiences. During REM sleep, while our eyes flicker and our muscles relax, exactly what the brain is doing is something of a mystery. But it is a type of sleep seen across the animal kingdom, in mammals and birds and even lizards.