Juggling is a term used in regards to fighting games, including the Street Fighter series, to describe the act of striking an airborne foe multiple times.
Juggling is generally defined as the act of striking an airborne opponent in such a way that they are sent flying again. This usually results in the opponent "bouncing" up and down with each hit, which earned the term its name.
Juggling is essential for setting up combos, and is a relatively easy concept to grasp and execute. However, long juggling combos may take some time to master, due to requiring knowledge of what a character's moves can do. Specifically, a player must know exactly what moves will launch the foe into the air, and which moves will keep the foe in midair long enough to allow another strike. Also, as with most combos, a player must know which combination of options will maximize damage.