Greylisting is like magical glass for mystical flies. Real email is delivered by real flies, whereas spam email is delivered by fake mystical flies. The difference is that fake mystical flies don't bang their head against the glass repeatedly when they hit a window to try and get through, they just bounce of and don't try again. However real flies do bang their head against the window repeatedly until they get through.

Now greylisting is like magical glass that can recognise real flies from mystical flies because it can see that this sort of fly (since flies are all numbered as we all know) is willing to bang its head against the glass repeatedly. As a result the first time it sees a new sort of fly it tests the fly - does it bang its head repeatedly to get through or does it give up. If it gives up, it will never let that fly deliver its message. However if it proves its worth as a real fly and retries, then the magical glass will, from that day forward always allow that fly (they're all numbered remember) through to deliver the real email it carries.

Now obviously, the first time a new fly is seen this also means the magical glass has to check to see if the fly is a real fly or a mystical fly. This takes a little while, so the first time this happens this can cause the email the fly is delivering to be delayed, but does reduce the spam you recieve, so its not all bad :-)