A radial is a circle, with 360 degrees spaced out around the circle. These degrees represent compass directions and relationships to other objects. Navigation aids are basically beacon radio stations. When you fly, you rarely can make radio contact with your destination airport. Navigation aids help you move between waypoints and also align yourself to approach airports.
Some aids carry voice communication, while others just transmit signals.
Using the degrees, you can set courses from navigation stations to move on to your next course. I'm not going to get into the differences, but for our purposes they're beacons that function to point us in the right direction. As an example of how to use navigation aids you might use one to set a course to a near by airport. Airports are where you obviously land and take off. Landing at airports can be tricky.