With cleated pedals (aka "clipless") it's how much wiggle room you have when you are clipped in. Some pedal systems will let you lock your foot into one spot (no float) and some won't lock you in at all (lots of float, like in Speedplay Frogs). There are a bazillion variations of float and you can adjust the tension and float on a lot of the pedals to exactly what you want.
I have Speedplay Frogs, and the lack of any tension and having tons of float makes my grumpy knee and hip very content. (no moving parts in the Frog system, and I love that kind of elegant engineering)
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