What is the priming option on a motorcycle fuel tank ?

it's a gpz900r . i can ride them but how they work is a mystery !!
Answers:
I'm assuming your bike's fuel tap has 3 positions - "Prime", "On" and "Reserve" ?

Usually, "On" & "Reserve" work off a diaphram which is activated by vacuum from the carb manifolds, so that when your bike isn't running, the fuel doesn't pour through the carbs and into the sump/your garage floor etc. . (A bit like a tap that closes automatically when the engine isn't running).

"Prime" works seperate from the diaphram, so that you can get fuel to the carbs when the bike's been sitting for a while.
On a GPZ900R Kwacker, the prime position on the fuel tap allows fuel to flow into the carburettors. Normally fuel only flows when the engine suction pulls the fuel diaphragm to one side, when the engine is running. When the carbs are dry (i.e. the engine has been standing unused) the diaphragms would prevent fuel flowing to 'prime' the carbs. So, set the tap to 'Prime' until fuel has filled the carb float bowls. Then set to 'Open'.