Well, I'm not talking about the place your friends took you to get over ol' what's her name. I'm talking about that mysterious pump bar rating you see in the home espresso machine ads.
1 bar = 14.5 psi
When a machine is said to be capable of 15 bar, it's capable of producing 218 psi. Since CW holds that espresso is best made using 9 bar (131 psi), you can see why some consider the pump rating as meaningless. What's important is whether the pressure can be regulated so the pressure used for brewing is at the proper level as the water hits the coffee. That's where an adjustable pressure relief valve (or OPV) comes in. The OPV lets you reduce the pressure in the group (using a gauge) to the proper level.
Pressure is just one variable that must be controlled in order to produce quality espresso. I'll go into the other variables at another time
Enjoy!