The most important position in Brazilian Jiu-Jitsu (BJJ) is the Guard position.
Most fundamentally the Guard is a position where you are on your back and using your legs to manage distance, control your opponent, and defend yourself. A working knowledge of the Guard position is absolutely essential for self defense, because if you're on the bottom in any other position you're definitely losing the fight. The Guard, however, is the tool you need in order to survive being on the bottom long enough to figure out another way to win the fight.
For skilled grapplers, however, the Guard transforms from a purely defensive tool to an offensive weapon they can use to win the fight from the bottom.
The Guard gives you many options to control you opponent so thoroughly he can never launch his own effective offence. Or to sweep him so that you end up on top and now he's the one on bottom in a terrible position. Or to finish the fight by submitting him with a variety of chokes, armlocks and leglocks.
The problem with the Guard is that it is a very diverse position. There are many different versions and variations, not all of which work in every context and for every body type.
A Glossary of the BJJ Guard by Stephan Kesting shows you all the most common variations of the Guard position, explaining where and when each position is most useful.
Never be lost when it comes to the BJJ Guard - download this book today and start figuring out how to win fights, matches and sparring sessions from the bottom!