The red LED consists of a white LED covered with a red filter. What does the red filter do?

The filter absorbs all colors of light except red. So if there's any red

included in the light going in, then the light coming out will be only red.

This is a big part of the reason why the filter looks red, and why it's

called a "red" filter.

(If there's NO red included in the light going in, then there won't be

any light coming out.)

The red filter changes the color of the white light. For example, if you have a white light, and then cover it with a green filter, the light is now green.