Pedals

what does float mean in describing the pedals?

The amount of movement you may feel in your heel, float is degrees of movement around the cleat, so if something had 6' float you can move your heel 6' before popping out of the pedals.

Some float is good for comfort, too much float allows you to screw your knees up. Road racers will prefer minimal float as they will have set their cleat angle for the optimum position for their knees, MTB riders prefer a bit more float to help them through technical things and have comfort.

Just found this which explains it better:

What is “pedal float”?

        It means the amount of rotation you can do with your foot before you              start to engage the release mechanism. Since we're talking rotation,              float is in degrees.  Generally speaking, float is limited by the              release springs in the pedal mechanism.

        The reason float is used is that the body doesn't spin the cranks              like a machine. There's an interplay between your hip, knee and ankle              that allows you to use your many muscles efficiently. So as your knees              go up and down, the complicated system of muscles and tendons is constantly              adjusting. For most people, this translates into some foot rotation              as the crank completes a revolution.

        It's related to individual body mechanics - that's why no one can              tell you how much float you need.  Shimano SPDs with “standard"              cleats have 4 degrees of float, Time ATACs offer 10 or 15 degrees.               Warning signs of inappropriate “float” for you are usually knee pain              (barring seat height issues) and a feeling of not being comfortable/clipping              out unknowingly.  Skill only plays a role regarding float in the acclimatization              stages of learning to use clipless pedals. 

        If you're happy with your pedals, you don't have to concern yourself              with float. If you're having pain, test ride a pair with more (Time              ATACs, for example) to see if it alleviates the problem.