How does meditation help in depression?

Imagine you have a rubber band in your hand. You start pulling it harder and harder until it stretches to snapping point. Depending on the strength of the band, it may or may not snap but the extreme tension cannot be denied. That is our brain under stress. Turns out, we're under constant stress of some kind or the other from the moment we wake up to the moment we fall asleep. Ultimately some snap. Some don't.
(You're probably already aware how many different kinds and qualities of rubber bands there are.)

When we realise that any further stretching is going to be the end of the band and we stop stretching the band further, thereafter trying to reduce the stretching little by little. That is our brain on embarking on meditation. Please note: the key here is to refrain from further stretching and slowly resorting to reducing the stretching.

Now imagine the same rubber band laying stationary on the table without being stretched. That is our brain when we have reached advanced level in meditation.

Note: It isn't easy to reach that advanced stage. It takes years and years of practice (so does also becoming a CEO!), but it is totally totally worth it. The amazing thing is, when you reach that stage you realise the whole world is one big ball of rubber bands. And you are playing with that ball.
Another very fitting example will be a trampoline.

Please remember: Regardless of what your doctor, psychologist or meditation course tells you: Meditation is NOT about stress relief. It is NOT a mental exercise to slow you down and become aware of your breathing. It is NOT a therapy to help you deal with your tension, depression, anxiety and mental pressure. Meditation is all about getting in touch with your inner self, the real 'you' that you don't even know how powerful it can be.

The relief from stress, anxiety, depression, mental pressure and other illnesses are just bi products of meditation.