Transfer roasted chiles to a large bowl and cover tightly with a pot lid, aluminum foil, or plastic wrap. Let chiles sit at least 15 minutes or until cool enough to handle. This allows the chiles to cool so you won't burn yourself peeling them, gives the flesh and skin more time to get used to the idea of separating and makes them easier to peel, and steams the chiles a bit to soften them more.
Note that the resting part is the key here. You don't need to cover and steam the chiles, as long as you let the chiles sit and cool for about 15 minutes, they will be easy to peel. The covering and steaming will continue to cook and soften the chiles, though.