Float refers to how many degrees your heel can move side to side when clipped into the pedal. Depending on your pedal brand of choice, you can buy fixed cleats with zero degrees of float all the way up to 15 degrees with Speedplay (which can also be adjusted).
While some pro riders talk about the benefits of being 'locked in' with a zero float cleat, the majority of people's joints benefit from some degree of movement. Most pedal systems will have a bit of friction built into the float while others such as Speedplay have a resistance-free feel to them — which is right for you will depend on your personal preference, but the best pedals for road bikes will come with cleats that offer a middle-of-the-road float.