A coating that can make certain species of Pokémon evolve. It is a special metallic film.
Special evolution items like the Metal Coat first arrived back with the second generation of Pokémon, and are now required when evolving Pokémon such as Scizor and Scizor and Steelix and Steelix. When evolving these Pokémon, this item is required in addition to the candy, and is consumed when used.
How to get the Metal Coat
The Metal Coat is obtained from spinning Pokestops and Gyms. There is a very low drop chance of around 1% per spin to receive a Metal Coat or any of the other evolution item, and there is an equal chance of obtaining each of the 5 currently available items.
There's no way to ensure that the Metal Coat will drop, but with only 4 other items, there's a greater than 50% chance that you'll receive one once you've spun over 250 Pokéstops.
The 7th day streak bonus gives you a very high chance of obtaining one of the evolution item, but it's not believed to be guaranteed.
Other evolution items
The other evolution items that can be found in Pokemon GO are: