Or maintain your enemy in front of you and you keep them from getting behind your back. In BJJ, it is best to keep your opponent before you. This is better than having your opponent on your side or behind you.

No absolute solid rules in the sport and martial art of brazilian jiu jitsu.  This is not an all standing rule, but it will help many beginners.  Most of the time, you want to keep your enemies in front. If you are in the guard position (top or bottom), Mount, side control, etc it is best not let your opponent move away from being in front of you. If they are moving away from being directly in front of you, it is a sign that they may pass your guard, sweep you, or worse - take your back.

This is especially important when trying to get out of a bad situation. If under your opponent it is better to turn your side to your opponent than it is to turn away or else you give your back. Keep in mind that there are techniques which can temporarily turn the other way or give your back for a very short moment.

This is true in many grappling sports, not only in side of brazilian jiu-jitsu if grappling with the bjj gi or doing Brazilian Jiu Jitsu with out the gi, wrestling, sambo and more.