Float is how much lateral movement you can make with your foot while clipped in without actually clipping out. It's measures in "degrees." As a practical matter, it allows your knee a bit of wiggle room in how it tracks. For some people, this float helps with knee pain. For others, it makes knee pain worse. With no-float pedals, it's very important to get them set up correctly. Of course, I'd argue that it's also important to get pedals with float set up correctly, too, but they arguably offer a wider margin for error.