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I'll try and explain. Okay say you had an open head. Just ports and then the air. As soon as the exhaust gases left the head they would be at atmospheric pressure. Now as an exhaust gets more restrictive, the gases try to move out quickly but can't move fast enough, and pressure starts building upstream. Like when you put part of your finger over the end of a water hose to make it shoot harder. You restricting the flow and causing back pressure. The pressure of the water behind the restriction grows, because it can't escape quickly enough. Backpressure is just the pressure that builds up behind restrictive parts of an exhaust. Is that a god explaination? I suck at explaining things sometimes.
-matt
EDIT: They have testers you can get for exhaust back pressure. They screw into your o2 sensor bung, and measure usually between 7-10 psi. You're just measuring how much pressure is built up in the exhaust, due to restriction.
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