It means that there's something called the REM sleep rebound. If you don't sleep enough or you don't get enough REM sleep on, say, Monday night, Tuesday night when you go to sleep, provided you didn't blitz your system with caffeine, you're not ingesting anything that would disrupt your REM sleep, such as caffeine late in the day, or alcohol, which will disrupt REM sleep dramatically, well, then you will get more REM sleep on Tuesday night.