hi Ashley, bar is a measure of air pressure see - http://en.wikipedia.org/wiki/Mbar
Its the pressure that the turbo is forcing air into the engine (forced induction)
most normal setups on turbo a Impreza, (standard) would operate up to 1 bar of pressure.
To give you an idea, a car tyre on say your mini should be 28psi that is approx 2bar of pressure.
The more air you can force into an engine the faster it will run.
But force more in than the engine can handle and it could all go BANG!
Hope this helps,
John