Siphon Mana works two ways.
Option A) You drain from an enemy and replenish your own supply while robbing his. Good for int heroes or heroes that have very little mana and you want them to keep them from casting more spells.
Option B) You click on an allied hero with siphon mana. This will take YOUR mana and give it to him. The mana supply can exceed the maximum, but it will burn off fast if you don't use it.
Option C) As dream_walker said, mana potions for when you need them.