Brainwave entrainment is a rather long name for something quite simple and natural. It is completely safe when used by a person not subject to a medical condition such as epilepsy.
The “entrainment” part just means that the rhythm that is played is picked up by the brain which synchronizes with it. This allows us to create audios that have a rhythm that we know can be useful for improving various conditions or reaching desired goals.
You may have heard about binaural beats. These are the best known of the ways in which brainwave entrainment is created. There are also the less well known isochronic tones which in fact are even more effective than binaural beats although I think they don’t sound quite so nice to listen to.
I have posted an in depth article on brainwave entrainment by Timothy Thorpe. If you want to learn more about this topic.