In Brazilian Jiu Jitsu, Judo, and Submission Wrestling, the guard is a position when you are on your back with your opponent standing or kneeling in front or over you, but has not passed your legs (ie, he still needs to get past your legs to advance). Brazilian Jiu-Jitsu (BJJ), also known as Gracie Jiu-Jitsu (GJJ), is a martial art that was developed in Brazil by the Gracie family during the mid-20th century. ... Judo (Japanese: 柔道 Jūdō; literally gentle way) is a martial art, a sport and a philosophy which originated in Japan. ... Submission wrestling (also called submission grappling or submission fighting) is a general term describing martial arts that focus on grappling to effect a submission (admission of loss) usually by means of chokes, joint locks, and other manipulations of the opponents body. ...

There are several different guards depending in which position your legs are, if your opponent is standing etc.

The most common guard taught is the closed guard: You are on your back with your legs around your opponents waist and your feet are crossed to hold the opponent tightly in order to control him/her and set him/her up with sweeps or submissions.

The open guard is the same position without the feet crossed, however pressure is usually still applied through the legs, this can often be done without the opponent realising the feet have opened.

Half guard refers to when only one of the opponents legs is trapped, usually by both of your legs, and while it can be used to control one of the opponents legs, it is more commonly used as a transition hold to set up sweeps and reversals.

There are also the spider, butterfly and hooking guard which are various methods of using the guard concept.