The very brief answer is:
Meditation Machines use a sophisticated form of audio and visual technology that uses light and sound to guide you gently and automatically into a deep state of mind and body.
A longer, more technical answer would be:
There is a lot of electrical activity in our brains and this activity forms our brain wave patterns. Meditation involves a slowing down of our brain wave patterns from the 'beta' of normal consciousness to slower rhythms known as 'alpha' and, in deeper meditation, 'theta'.
A Meditation Machine makes use of light and sound to slow down your brain wave activity and by doing this, they take you gently and easily into a state of meditation.
In this way, they are utilizing your brain's 'frequency-following response', which is your brain's natural and automatic tendency to follow the frequency of light and sound.
A Meditation Machine consists of a small, hand-held unit, stereo headphones and special glasses inset with light-emitting diodes (LEDs). During a session, these lights flash and are synchronized with specific sound patterns that are heard through the headphones.
After a short time, your brain waves begins to resonate, or entrain, at the same frequency as the light and sound. This is similar to the relaxing effect you might experience by gazing at a gently flickering flame or listening to soothing music.
The tone, frequency and other parameters of the light and sound stimulation vary the effect and Meditation Machines have many different types of sessions built into them for relaxation, meditation, stimulation, creative visualisation and so on.
In addition to entrainment, the imagery created by the light and sound provides a focus for your mind and quiets internal dialogue or chatter. The various areas of your brain begin to integrate into one whole, harmonious unit as experienced by masters of meditation. This entrainment and integration is passive and occurs naturally without your conscious effort.
You can learn more about Meditation Machines at
http://www.meditationsuk.com/meditation_machines/index.htm