A low Earthean orbit (LEO) is an orbit in which objects such as satellites are below intermediate circular orbit (ICO) and far below geostationary orbit, but typically around 350 - 1400 km above the Earth's surface. Orbits lower than this are not stable, and will decay rapidly because of atmospheric drag. Satellites in a LEO travel at ca. 27,400 km/hr (8 km/s), that is one revolution in ca. 1.5 hour.
A low earth orbit is a stepping-stone to travel beyond orbit but it is also very useful for communication satellites because of its proximity to Earth. It requires less energy to place a satellite into a LEO and the satellite needs less powerful transmitters for data transfer, the downside is that a network of satellites are required to provide continuous coverage. Remote sensing satellites are also often in LEOs because of the added detail that can be gained.
The LEO environment is becoming congested, not least with space junk. The United States Space Command tracks more than 8,000 objects larger than 10cm in LEO.