Direct light is simply light that reaches our subject directly from the main light source in our scene. It is a simplification, because it has to have been reflected by our subject for us to see it, but a useful one.
Generally, the sun will be our main light-source during the day. Therefore, if the sunlight is incident on our scene or subject, they are in direct light.
With our exposure locked to these values in MANUAL MODE, we know that everything lit by the direct sunlight will be properly exposed; or very close.
We make a distinction, again quite arbitrarily, between Direct Light from the sun on a clear day and DIFFUSED LIGHT, which may still be sunlight, but having been diffused by clouds or haze.
Direct light is the opposite of REFLECTED LIGHT.