High Earth Orbit ("HEO") is a somewhat vaguely defined term which generally refers to an orbit around the earth that is located above geosynchronous orbit (
GEO).
Objects in High Earth Orbit are rarely in a stable orbit; they are usually 'lost' to solar orbit after a short time due to perturbations of the Moon, Sun and Jupiter.
Additionally, objects in HEO are easily able to enter the interplanetary superhighway and hence can travel to Mars and Venus and beyond.