That's a poorly written macro ;[ First off, you cannot cast Binding Heal on yourself, so an alt modifier is useless. Not only that, but that alt modifier doesn't target you, but a mouseover. The priorities are also wrong. A macro is read top to bottom, left to right. As such when you are compounding conditionals, the one with most priority is to the left while the one with least priority is to the right. As it is written, assuming you are using Flash Heal instead of Binding Heal, if you were to hold ALT, you still would not be casting it on yourself unless you did not have a mouseover target.
As a healer, you would want to prioritize mouseover over target. It should be written like this:
#showtooltip NAME OF SPELL
/cast [mod:alt,@player][help,nodead,@mouseover][help,nodead,@target][help,nodead,@targettarget] NAME OF SPELL
0 -Activate Gloves
1 -If I'm holding ALT, cast this spell on myself
2 - If 1 is not true, AND if the mouseover target is a friendly and not dead, cast this spell on that mouseover target
3 - If 2 (and thus, 1 as well) is not true (aka there is no mousover that is a friendly ATM), AND the target is a friendly and is not dead, cast this spell on the current target
4 - If 3 is not true, AND the target of the current target is a friendly and is not dead, cast this spell on that player. (This is for when you are targeting the boss, which should be 99% of the time when you are healing)
Replace NAME OF SPELL with the appropriate spells. In the case of Binding Heal, I suggest using an alt modifier to cast Flash Heal or use a Healthstone or Desperate Prayer.