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• Direct Lighting is when light from a source strikes an object directly without anything between the light and the object and seen by the camera. Imagine a flashlight on the wall, the sun on a sidewalk, or the light from this screen onto your desk. That's direct lighting.
• Indirect Lighting is when light has already interacted with others objects before reaching the camera. Imagine a mirror, the light is illuminating everything that is seen indirectly in the mirror (you're lit by a light that then bounces to the mirror and then into the camera, you're seeing the effect of lit objects indirectly in the mirror). Or a whole room can be lit up by a window where the sun is only shining on the floor. These are all indirect lighting examples. Huge amounts of information in our daily lives is indirect. Right now you may be in a room where fixtures cover the lights, that means everything you see is indirectly lit.

There is a funny thing about specular objects without diffuse shading, they only look like the things they reflect. If you were to render the Cornell Box scene examples with perfectly specular surfaces and a light, all you would see are reflections of the light. What does a mirror look like? Can you describe a mirror that has nothing to reflect? Or does the mirror look like what it's reflecting? This is why indirect results are so important for realism and ray tracing.