Air navigation is navigation while flying. It is used by pilots in aircraft to know their exact position and find their way. That is important because if they get lost, they can hit a mountain or fly into a dangerous area or not find a place to land. There are generally two types of air navigation depending on weather. In good weather, pilots navigate themselves visually with maps. But when the weather is bad and they do not see the ground, they use special radio navigational instruments or the air traffic controller navigates them. The first kind of navigation is called VFR (visual flight rules) navigation. The second is IFR (instrument flight rules) navigation.
When flying under VFR, or Visual Flight Rules, pilots navigate using purely what they can see outside their aircraft, sometimes using a special map called a sectional. When preparing for a flight they prepare a route going between features labeled on the sectional map, including things like roads, and large structures like windmills. Pilots when flying with VFR are responsible for avoiding other aircraft and terrain.
When flying under IFR, pilots fly set routes only, following airways and tracks between physical radio beacons and waypoints. While the pilot is still responsible to avoid incident, navigation under IFR is heavily reliant on an Air Traffic Controller's instructions.