For example, exercise and perusing a goal very ambitiously are both types of stress, except they ultimately benefit the body. Areas of the brain, including the hippocampus, amygdala and prefrontal cortex can pick up on positive stressful experiences and cause “stress-induced structural remodeling” of the brain, which means you experience alterations in behavioral and physiological responses to these positive events. The result is that in the future you’re better able to handle similar situations because you learn from them, associate them with a reward and stop perceiving them as threatening.
Cortisol is the principle hormone (although not the only hormone) tied to our innate “flight-or-fight” response, which is how the body reacts to acute stress by either helping us run from the situation or stick around and fight our way through. When short spikes in cortisol/adrenaline happen over and over again nearly every day, they cause wear and tear on the body and speed up the aging process.
Theta brainwaves occur most often in sleep but are also dominant in deep meditation. Theta is our gateway to learning, memory, and intuition. In theta, our senses are withdrawn from the external world and focused on signals originating from within. It is that twilight state which we normally only experience fleetingly as we wake or drift off to sleep. In theta we are in a dream; vivid imagery, intuition and information beyond our normal conscious awareness. It’s where we hold our ‘stuff’, our fears, troubled history, and nightmares.
Stress can either be perceived as feeling good/positive or bad/negative depending on the context, and the body reacts differently to both kinds. However, where the body isn’t so clever is distinguishing between very serious threats (like being robbed or starved) and events that are stressful but not actually life-threatening. Unfortunately, whether a problem is very serious or not, the body usually has no way of knowing the difference− — anything that causes you to worry, anticipate, regret, overthink or panic can send your stress levels through the roof.
You listen to binaural beats using headphones. In each ear, you receive sound at a slightly different frequency (often accompanied by some relaxing background sounds). If your left ear receives a 300-hertz tone and your right ear receives a 280-hertz tone, your brain will process and absorb a 10-hertz tone. That’s a very low-frequency soundwave—one you can’t actually hear. But you don’t need to hear the sound for your brain to be affected by it.
A common element in recordings incorporating alpha and theta frequencies is a steady but barely perceptible rhythm of the frequencies themselves. This subtle and calming pulse mixes with sounds of gentle breezes, distant bird songs, and the slow progression of deep synth notes. Underneath this, below the audible sounds at sub 16 hertz levels, other frequencies intermingle, deepening the merging of conscious and unconscious mind.
When you play two separate frequencies in each ear, say 140 Hz and 149 Hz, the difference in frequency is 9 Hz. Your brain compensates for this difference and a third tone of 9 Hz is experienced. This will result in your brain being lowered or raised and tuned into the desired frequency. This is how binaural beats are produced. Headphones are essential for this to work.  
There's no doubt that brainwave entrainment audio is a great technology for anyone who is interested in encouraging altered states of consciousness and it certainly can help people to reach deeper states of relaxation than they might normally have access to. I myself will often listen to brainwave entrainment music in order to enhance my practice of meditation. But let’s keep a balanced perspective, put all the marketing hype to one side for a moment and acknowledge that there is no brainwave entrainment technology in this world that can make you meditate like a Zen monk “at the touch of a button”, despite what some might like you to believe. The mind is NOT a machine. It moves through various states of consciousness in an organic way and at a natural pace. We certainly can guide and accelerate that process with the use of brainwave entrainment audio, but we cannot control it with the same sort of specificity and immediacy as you might control the speed of the car you drive.

If you’re going to enter a state of meditation, a technique of some sort must be employed that takes you there. (Note – I am fully aware of references to “spontaneous enlightenment experiences”, but these are an entirely different category of phenomena than what is being discussed here and are outside the scope of this discussion. Perhaps another time.)
Over-arousal in certain brain areas is linked with anxiety disorders, sleep problems, nightmares, hyper-vigilance, impulsive behaviour, anger/aggression, agitated depression, chronic nerve pain and spasticity. Under-arousal in certain brain areas leads to some types of depression, attention deficit, chronic pain and insomnia. A combination of under-arousal and over-arousal is seen in cases of anxiety, depression and ADHD. more...
Subsequently, the term 'entrainment' has been used to describe a shared tendency of many physical and biological systems to synchronize their periodicity and rhythm through interaction. This tendency has been identified as specifically pertinent to the study of sound and music generally, and acoustic rhythms specifically. The most ubiquitous and familiar examples of neuromotor entrainment to acoustic stimuli is observable in spontaneous foot or finger tapping to the rhythmic beat of a song.