Hexes are the standard unit of measurement in Fallout, Fallout 2, and J.E. Sawyer's Fallout Role-Playing Game. Fallout Tactics and Van Buren also technically used the hex, though the maps were not divided into hexes, and hex distance changed based on stance and Agility score.
The entire game map is made up of hexes. When not in combat, you can freely travel across a hex. During combat, though, each hex costs one Action Point to move across. With a Crippled Leg, each hex costs 4 AP to move across, and with two crippled legs, you can't move.
A weapon's maximum range is measured out in hexes. A melee weapon or an unarmed weapon usually have a range of one hex, so you need to be right next to something to hit it. A spear or a sledgehammer, though, have a range of two, so you can hit the enemy from a little bit farther distance.
A gun has a range that is somewhat determined by the ammo type and power of the gun. A pipe rifle, for instance, has a range of 20 hexes, while a Colt 6520 has a longer range at 25. The longest range for weapons is 50 hexes, possessed by the DKS-501, the PPK12, and the M72.
In Fallout Tactics, the hex still appears, though not in a literal sense. Moving costs a number of action points based on the distance, and what stance you are using. Some characters, like deathclaws, will move faster in CTB than in ITB or STB, as well.
In Fallout and Fallout 2, you can't actually scroll the screen far enough to make use of the 50 hex range on the weapons.