There are four types of brain waves, indicating a range from most to least activity. This is not a detailed anatomical explanation but should give you a basic understanding.
Beta Waves...When very actve, the brain generates beta waves. Beta waves are of relatively low amplitude yet, at 15 to 40 cycles persecond, are the fastest of the four different brain waves. A couple engaged in an argument, a person feeling stressed over a missed deadline, a student experiencing pre-exam stress, and a parent irritated by a crying child would exhibit beta waves.
Alpha Waves...Alpha waves represent the amount of brain activity in a relaxed state. Alpha waves are slower than beta and higher in amplitude. They range from 9 to 14 cycles per second. A businessman settling down to relax after a presentation at a major conference would most likely exhibit alpha waves. So would a parent relaxing with a comedy movie after putting a crying toddler to bed.
Theta Waves...Theta waves are even higher in amplitude and slower in frequency than alpha waves, at 5 to 8 cycles per second. You will probably exhibit theta waves if you start visualizing the vacation you plan to go on in a few weeks. Many people experience theta waves while enjoying a long, luxurious bath.
When we sink into the theta state, we access more-creative parts of the mind, which can take many complex ideas, theories, and data and turn them into a revelation. One of the best-known revelation that occurred in this state was Archimedes' discovery.
Delta Waves...Delta brain waves have the greatest amplitude of all four states and the slowest frequency, at about 1.5 to 4 cycles per second. If your brain activity were zero, you would be brain dead.
Someone in a deep meditative state could have a significant proportion of alpha, delta, and theta waves, but only some beta waves.