What is “focus”?
Focus is a UnitID like target, player, or raidpet1target. It allows you to reference a mob, player, or NPC you specify. The simplest usage of focus is with keybindings. There are two focus-related functions in the bindings menu: Focus Target, and Target Focus. Focus Target sets your focus to whatever your target is or clears your focus if you have no target. Once you have a focus set, you can use it as a unit ID for any other command. Target Focus will, as you might guess, target the entity you have focused. However, this doesn’t necessarily take full advantage of focus. One of the most common uses is to set a crowd control target. A mage can select a mob to sheep and set it as their focus.