A source of light is an object that send out light.

eye

light source

In the diagram you can see a line if light shooting into the eye.

Light is something that lets you see. For example:

In the school cloakroom the light is automatic so when nothing is moving the light turns off,but when you look at the cloakroom when the light is off you can still see the coats. This is because there has to be a little bit of light so you can see.That is why when you stand near or under a light you can see the objects around you clearly.

How we see?

shadows are made when an object blocks the light.