An egg floats in water when the air cell inside it has grown large enough to make it buoyant, which mainly means the egg is old and of lower quality. It does not by itself mean the egg is spoiled: the U.S. Department of Agriculture says a floating egg may be perfectly safe to use.
A freshly laid egg has almost no air cell. As it ages, moisture and carbon dioxide escape through the pores in the shell and air replaces them, so the air cell at the large end keeps growing. That is why fresher eggs sink while older ones can float. To decide whether a floating egg is still good, crack it into a separate bowl and check it; a spoiled egg has an unpleasant odor, raw or cooked.
Source: USDA Food Safety and Inspection Service, “What does it mean when an egg floats in water?”
Image: Brown chicken eggs by Justus Blümer, CC BY 2.0, via Wikimedia Commons.