Medium Earth orbit comprises a wide range of orbits anywhere between LEO and GEO. It is similar to LEO in that it also does not need to take specific paths around Earth, and it is used by a variety of satellites with many different applications.
It is very commonly used by navigation satellites, like the European Galileo system (pictured). Galileo powers navigation communications across Europe, and is used for many types of navigation, from tracking large jumbo jets to getting directions to your smartphone. Galileo uses a constellation of multiple satellites to provide coverage across large parts of the world all at once.