Entrainment is like priming the pump of meditation. It doesn’t “cause” meditation any more than pouring that little bit of water into the pump initially “causes” water to appear in the well. The water was already there. The conditions for the pump to do its job just needed to be set up. What entrainment does is to create the conditions and the internal environment typically associated with meditation. You could get to that point without entrainment, certainly, but the entrainment allows you to get there quicker, more consistently, and to remain there for longer periods of time than you would ordinarily be capable of doing on your own.
Delta brainwaves are slow, loud brainwaves (low frequency and deeply penetrating, like a drum beat). They are generated in deepest meditation and dreamless sleep. Delta waves suspend external awareness and are the source of empathy. Healing and regeneration are stimulated in this state, and that is why deep restorative sleep is so essential to the healing process.
Gamma brainwaves possess the highest frequency we know of. In this state, our brainwaves are so harmonious that many people report feeling elevated states of consciousness, even spiritual experiences. This frequency is often experienced by monks, nuns, yogis, and experienced meditators. Gamma frequencies also enhance memory, awareness, and mental processing.
Another very useful application for the Theta & Delta frequencies is to keep your eyes open while listening to the entrainment recordings. Simply pick a spot on the wall and look beyond it as if you were looking at the view beyond the wall. Keeping your eyes open while in Theta and Delta brainwave states stimulates connections between the visual information processing centers of your brain and the "dream vision" centers that normally only function with the eyes closed.

For example, if a 530 Hz pure tone is presented to a subject's right ear, while a 520 Hz pure tone is presented to the subject's left ear, the listener will perceive the auditory illusion of a third tone, in addition to the two pure-tones presented to each ear. The third sound is called a binaural beat, and in this example would have a perceived pitch correlating to a frequency of 10 Hz, that being the difference between the 530 Hz and 520 Hz pure tones presented to each ear.

Move your focus to the sole of your right foot. Tune in to any sensations you feel in that part of your body and imagine each breath flowing from the sole of your foot. After one or two minutes, move your focus to your right ankle and repeat. Move to your calf, knee, thigh, hip, and then repeat the sequence for your left leg. From there, move up the torso, through the lower back and abdomen, the upper back and chest, and the shoulders. Pay close attention to any area of the body that causes you pain or discomfort.

Now I’m in the market for two extra tracks to have some variety. I have narrowed it down to a couple of ones including muscle relaxation which is theta and delta. I have a hard time relaxing my body. The last time my muscles where relaxed was after intensely cleaning my stove. I don’t want to clean every time I want to relax, even though my house will be spotless ;).
Brainwave entrainment is a colloquialism for such 'neural entrainment', which is a term used to denote the way in which the aggregate frequency of oscillations produced by the synchronous electrical activity in ensembles of cortical neurons can adjust to synchronize with the periodic vibration of an external stimuli, such as a sustained acoustic frequency perceived as pitch, a regularly repeating pattern of intermittent sounds, perceived as rhythm, or of a regularly rhythmically intermittent flashing light.