Basically, when at idle, your engine will have to suck in the air, so you should be getting a negative value. Once you tap the throttle, the needle should jump to 0, or close to it. Once you start to accellerate you should show positive boost. You can use this thread to help you convert the boost shown on the stock gauge to PSI.
So basically the boost gauge tells you how much vacuum or boost you have.Quote from PlatinumWRX in said thread:
"0.1 MPa = 1.0 bar = .987 atmospheres = 14.5 psi = 29.53 inHg"
More you accellerate, more exhaust gas spins the turbo, more air is compressed into the engine, higher boost readings on gauge.
On a stock car you should see a max of a little below 1.0 bar i believe, around 13.5 psi. You also must have a boost gauge to tune in your MBC if you choose to get one.