Float is the ability of the cleat (and the shoe it's attached to) to move or rotate laterally without clipping out of the pedal. There are various kinds of float, too - lateral as well as rotational. higher float generally means more - your foot will roate or move more before being constrained. The original clipless pedals did not have float - your foot was locked in to one position.

The red cleats have the MOST float of the Look Keo cleats (9 degrees), the grey ones are in the middle (4.5 degrees) and black cleats have no float (0 degrees, or fixed). For Look pedals, the float is primarily rotational - at least until the cleats start to wear down!

In a perfect world, you'd find a minimum amount of float you need for your peddling style, and if you had more it wouldn't matter. In the real world, though, very few of us are pushing straight down on the pedals, and lots of cyclists don't like the sliding on ice feeling you can get with lots of float.

As a practical matter certain Speedplays have the most float of commonly available cycling pedals, wtih the added benefit of being able to adjust the amount of float without changing cleats.

FWIW I use Look Keo Sprints with red cleats, sometimes grey ones. Both work fine for me, but tyring the ones with no float was a problem for my knees.

JB