I had a PM asking me to explain this, and Jedroth asked me to explain this as well on the Warrior forums. This was my response to the player. Hopefully this explains for those curious:

/script SetCVar("targetNearestDistance", 10)
/targetenemy [dead]
/targetenemy [noharm]
/script SetCVar("targetNearestDistance", 41)

*First, the macro sets your maximum target distance to 10 yards (melee range for special attacks).

*Then, if your current target is dead, it targets a new enemy (just like your normal "T" or tab button would) within 10 yards.

*If your target is not dead, but cannot be harmed (is another player, or a friendly NPC, etc), it will target a new enemy in 10 yards.

*Then, so you don't get screwy results in general tanking, it resets your max target distance back to normal (41 yards).

It's basically a failsafe for when you kill a target, so you don't waste a cooldown before you are on another target. I wrote it specifically for my Shield Slam/Trinket macros so I would max out overall damage from the trinket use (if you macro and time every one perfectly, you get 4 Shield Slams instead of 3). However, it's also a handy failsafe for other attacks and abilities, so I put it in there.