The guard is a common position used in Brazilian Jiu-Jitsu and ground-fighting. Your opponent is said to be in your guard when you have your legs wrapped around them. Although you are on your back you are actually in a more dominating position.
Joint locks and chokes can be easily applied from this position.
With your legs crossed behind your opponents back (closed guard), you can straighten your legs to crush their chest, this can be very uncomfortable for your oppoenent and obviously make it hard for them to breathe.
Being in someones guard is not as severe as being mounted, but there are not many techniques that will defeat your opponent. Do not waste time attempting techniques whilst in someones guard, escape and attain a better position such as the mount and then apply submission holds.
The simple 'closed' guard. Keep your head on the floor to save energy in keeping your head up. Obviously if someone is throwing strikes DON'T leave your head on the floor since you'll be knocked out in seconds with nothing to cushion the blows.
For beginners it is best to always use the 'closed' guard to stop your opponent escaping easily.
Note the hand positions used to keep control of your opponent and ready to apply numerous submission holds.