This is the most important punch in boxing. The purpose of the left jab is to keep your opponent on the defense and to accumulate points. If you are keeping a jab in your opponents face, he will have to concern himself with defense which will afford him less time to work on his offense (hitting you in the nose).
You can use the jab to disrupt your opponent’s rhythm and timing, in addition to using it to control the bout.

The left jab is thrown by extending the left hand and arm fully, keeping to left elbow down and turning the forearm so that the knuckles land in a horizontal position. Speed is derived by keeping the elbow down and snapping the arm forward at the elbow joint. Power is possible by slightly pushing the body, particularly your left shoulder, forward with your right foot and leg when the jab is thrown.