The auto exposure on digital video cameras is generally very good at setting the correct lighting. And most difficult lighting situations should be solved first by changing the position of the camera or the subject - such as not shooting into direct sunlight.
But there are occasions when you'll need to manually adjust the exposure on your camera.
One example is on a bright day where there's lots of movement and light contrast in front of your camera, such as buses passing by with large billboards on their sides that reflect the bright sunlight. The camer then will open and close its exposure in response to these changes.
Or if you have to take a shot of a person from a certain angle, and there is bright light behind the person.
In these cases, aim your camera at the light setting you want for your shot and then switch from auto to manual exposure.
For example, if you're shooting an interview with someone, zoom in on the person's face, hold the shot there and then switch from auto to manual exposure.
The camera then will retain or lock in whatever setting you selected throughout your shoot, despite any changes in the lighting.