They're like skittles, and in the movie Last Man Standing (coming soon), based off of the graphic novel of the same name (coming sooner), these candies give small power ups and boosts. It's a very fun sci-fi universe. I'd recommend following Dan LuVisi, the creator of LMS. You can probably even friend him on Facebook, I did a few years ago. He's just a cool, funny guy, and craaaazy talented. Last Man Standing is gonna kill it.
Hero number thirty-three,
Was clobbered by a walking tree.
Hero number thirty-four,
Collapsed before he reached the door.
And hero number thirty-five,
Opened a chest that was alive.
Hero number thirty-six
He fell before a wizard's tricks.
Hero number thirty-seven
Now resides in hero heaven.
Hero number thirty-eight
He saw the boulder just too late.
And hero number thirty-nine,
Was run right through by Ornstein.
And hero number forty, well,
A giant wolf is how he fell.
You'll soon be breathing your last breath,
This isn't life, it's living death.
And though you've only just begun,
You'll soon be hero forty-one.
Mettle has its origin in a spelling variant of metal used in metaphors.
Mettle and metal are basically two different ways of spelling the same word back during Shakespeare's time, when English was a smelting pot and there were fifty different ways to write a given word.