An earth orbit that is lower to earth then both a medium and high earth orbit.

This is because of the Gravitational pull of the earth.

Gravity keeps satellites in orbit. The closer you are to the Earth, the faster you have to go to maintain your orbit.

At low Earth orbit, the altitude of the Space Station, you make an orbit every 90 minutes. At the Moon's distance you need over 27 days to go around the Earth.

In-between there is an altitude which matches the rate of the Earth's rotation. Many satellites orbit at this altitude.

With large number of low-earth-orbit satellites and the geo-synchronous orbit stuffed full of communications satellites, there aren't all THAT many "medium" altitude satellites. But there are some.

The GPS navigation satellites, for example, are in highly inclined 12-hour orbits, and an unknown number of military "spy" satellites are in that middle range.

Satellites orbit in several different regions of space in the sky. Orbits fall into 3 main categories based on distance: Low Earth Orbit (LEO) at an altitude of 160 - 2000 km, Medium Earth Orbit (MEO) at an altitude of 2000 - 35000 km, and High Earth Orbit (HEO) at altitudes greater than 36000 km. One special orbit in which a large number of satellites have been placed into is a geosynchronous earth orbit (GEO), which is at the exact distance such that the satellite orbits the earth at the same angular velocity at which the earth spins. Most satellites orbit in circular orbits. A few are in elliptical orbits in which their distance varies.