Here is a huge thread on the topic.
https://www.backyardchickens.com/t/644300/fermenting-feed-for-meat-birds
But in a nutshell, grains all grains have protective enzymes and anti nutrients designed to protect the seed and keep it able to germinate. These make digestion and nutrient retrieval by monogastric animals difficult. To bypass that natural seed protection you can ferment the grains. This makes the nutrition in the grain more bioavailable to monogastric animals. So your chicken gets more "bang for the buck" and more feed value to what you are already buying, besides which they love the stuff.
There are many, many formulas and additives and what nots you can read about on the thread, lots of people complicating and adding to the process, but grain (or feed that is grain based) + water + time = fermented feed. The feed is best when fully fermented and about 1+ weeks old and older. You just add more grain and water as you use it. Making sure your batch is big enough to feed out for several weeks before having to add new. No metal containers.