In essence, sound moves in sonic "waves". Lower frequency (bass) waves are spread out further (think more distance between crests of waves) than higher frequencies are. Now picture a wave, ~~~~~, a null is a point where that wave's "hill' or 'valley' is going over/under you. The result is you hear less of that sound. Meaning if you walk closer to your subwoofer the bass will seem to increase, but when you go back to the null the bass will seem to disappear.
Hopefully that description wasn't too far off or confusing.
Read this article on subwoofer location,
Crawling for Bass.
Good Luck
Jack