Easy way is just type it into google mate.

"1 bar in psi"

OR

"20 psi in bar"

As T28 say, 1 bar = 14.7 psi

psi stands for Pound per Square Inch (sometimes simply referred to as pounds, ie "Im running 20 pounds of boost"), this is simply a measurement of how much pressure is being produced by the turbo going back into the engine. The more you use the faster your car will be, but use too much and you will kill your engine if it hasnt been built for use with high boost, this is because hight boost can put alot of stress on the internals of your engine, and uprated parts need to be used to take full advantage of it.

Bar is just a different measuring method of psi

Stages of boost is usually referred to as Hi or Low boost. People will have a switch inside to flip from low boost (say 7psi) to hi boost (say 20 psi), this way they can drive normally on low boost, and when they wanna go a bit wild or chase after ferarri's they can flick the switch to "hi" and off they go!

Hope that helps a bit.