In
jiu-jitsu or
judo, 'the guard' refers to a fairly strong defensive position when
grappling.
An opponent is 'in your guard' when you hold them between your thighs while you lie on your back. Ankles may or may not be crossed behind your opponent.
Note that Royce Gracie impressed the martial arts world by consistently defeating opponents in the early Ultimate Fighting Championship tournaments from the guard -- many chokes and joint manipulations can be performed from this 'defensive' position.
Other grappling positions include the top mount, the side mount, the scarf hold, the north-south mount, the knee mount and the rear mount.