Elixirs are for enchanting.
Enhancement runes will protect an item up to +10. If you have a +9 and try to upgrade it using the rune (and make sure you do put it in the slot, it isn't automatically used just by having it), no negative effects will occur if it fails, you'll just lose the rune. And if it succeeds, you get to keep the rune. It can't be used when upgrading from +10 to +11 or higher.
Elixirs help increase the rate of which things can be enchanted. Enchanting is done with prefix and suffix
scrolls. There are also cash shop runes to prevent the negative effects of failed enchanting.