A prego (literally "nail" in Portuguese) is a simple steak sandwich served on a crusty roll. The steak tends to be tenderized or nailed hence the name.
The McDonald's version skips out on the steak and has a beef hamburger patty instead. The rest is simple: just a garlic sauce and a crusty roll.
There's also an option for an egg. Sounds good to me!
It feels a like a homegrown Portuguese version of McDonald's France's McBaguette.