Repair automatically heals all units on the tile by x amount, as long as they are mechanical (ships, siege engines, golems). It works pretty much like heal. However, it doesn't heal as much as the spell heal does. Also like heal, you can cast it as many times as you have units for - so if you have 1 adept with enchantment 1, you can cast it once, and if you have 5, you can cast it 5 times. The healing takes place the instant that it is casted. However, it only heals something like 5 or 10 hp, meaning that a unit that took 90% damage would need to be repaired a number of times to come to full strength.
-Colin