Where is the source of XNA lighting?

Talking BasicEffects, where is the source of the three directional lights?

When I write this:

effect.DirectionalLight0.DiffuseColor = new Vector3(0.5f, 0, 0); // a red light
effect.DirectionalLight0.Direction = new Vector3(1, 0, 0);


What does it mean for the direction? From where is it directing?