Heavy is the wake vortex category of that particular aircraft.
In other words the amount of turbulence that is created by the aircraft moving through the air.
The bigger the aircraft the bigger the turbulence caused therefore the distance a following aircraft has to follow is in relation to the previous aircrafts wake vortex category.
Aircraft of the same wake vortex can follow using the minimum distance.
Wake vortex is only a problem when aircraft are landing or departing, in en-route we have to use 5 miles anyway so wake vortex doesn't come into it.
http://www.chirp.co.uk/New/Downloads...99.pdf#search='wake%20vortex%20categories'
This link might help.