Support Range is a passive ability found on many weapons. It allows a character to join a fight from 1 hex further away.
Before a player enters an enemy hex, the game will show the combat range, with red hexes. Within these red hexes all party members and up to three enemies will take part at the combat. Some items will grant you support combat range, one additional (orange) hex to the basic area. The area actually depends on the region and the weather.