Slow wave sleep is the deepest level of sleep, occurring mostly in the first half of the night. It's called slow wave sleep because the waveforms 1-3 times per second - contrast this with non-deep sleep, where the waveforms occur 4-7 times per second. This article discusses research showing that sounds produced at the same frequency as slow wave sleep, and listened to while asleep, can enhance word associations. Sounds produced at different frequencies did not improve word associations.
The researchers also think these sounds may improve the overall quality of sleep - more studies are needed though.