A binaureal beat is created by playing a different tone in each ear, and the interference pattern between the slightly differing frequencies creates the illusion of a beat. It's intended to be heard through headphones, so there's no cross-channel bleed across both ears. Listen to this, I'll play a simple binaural beat, and I'll slide the pan control back and forth from one ear to the other. You can see that there isn't actually any beat, it's just an acoustic illusion:
All brainwave frequencies are useful and beneficial at certain times – there is no brainwave that is intrinsically better than another. However, by deliberately choosing to attain a particular brainwave state, a corresponding mental state can be brought about at the same time. For example, a working person who has been in an overly alert beta brainwave pattern for many hours can quickly shift their mind and body into a relaxed state by listening to a few minutes of brainwave entrainment music for inducing alpha or theta brainwaves.
If brainwave entrainment leaves you with unwanted side-effects (see below) or discomfort, you’re probably encouraging a range of brainwaves that are already excessive in some area of your brain. The way around this is to get a brain map to see what your brain’s strengths and weaknesses are, and see what (if any) brainwaves could use some encouragement.