A float is when a move hits an opponent out of an airborne state, such as during a hopkick or orbital. The opponent gets "floated" into the air, and if done with quick enough moves, this will lead to an opportunity for a juggle combo.
Due to the fact that basically any move can float an opponent, this mechanic adds a significant risk to airborne attacks and a benefit to poking opponents out that are attempting to use such moves to get out of your pressure. A 1 jab or df1 giving a full juggle is very strong given this situation, as they are far faster and less committal than an average launcher. To offset this, float combos do tend to scale harder than a regular juggle off of a launcher.