It means going back every three days to a particular location and killing everything in the vicinity, often with the underlying assumption that you're using the 'Wait' function to advance the game-calendar while in a different cell from the one containing the targets.

Some items are only dropped by certain critters/NPCs, so repeatedly killing a sizable concentration of same is the only reliable way of amassing a large quantity of said items. In the case of Fiends, there is a side issue of the Van Graffs' inventory having the bad habit of 'locking up' and staying at the same low-level crap it starts with; as a result, the Fiends become the de facto best source of standard Energy Weapons and ammo unless you have unlocked certain rep-dependent vendors.