In the absence of an electric field, the free electrons in the metal have random motions. During such motion, they collide with other atoms and electrons present in it again and again so that their direction changes. As a result, the net motion in any particular direction becomes zero and hence no current flows. But, when an external electric field is applied, the free electrons experience a force and start drifting towards the positive terminal of the source with a small drift velocity, and the current also starts to flow.