If you are holding a compass and facing north, the direction to the west of you is also to your left. Left is the opposite of "right."
People who are left-handed write with their left hands. and when you take a left turn on your bike, you should signal by holding your left arm straight out to the left. When left is an adjective, it has a completely different meaning: something that's left on the table, or remaining there, will probably be thrown away by your waiter. The Old English root is luft-, "weak," as most right-handed folks are weaker on their left sides.