Game terminology
In the context of video games, juggling is the act of hitting your opponent into the air repeatedly. Juggling is especially common in "fighter" games (for example: Soul Calibur, Tekken, Virtua Fighter, etc) and in some first person shooters where the physics allow it (Quake Team Fortress, Quake 3 Fortress, and a number of other fps's).
Juggling can also be applied to certain other situations. But basically, juggling is a series of attacks that virtually immobilize your opponent and give you a number of free attacks with which to hurt them.