It is called "Beef bowl" in some country.
"Gyu" means beef, and "don" means a bowl rice topped with some ingredients.
Gyudon is a bowl rice topped with thinly sliced beef and onion, simmered in mildly sweet flavored, soy sause based broth.
We usually put BeniSyoga(pickled ginger) on it, befor we eat it.
Gyudon is also popular as a dish of fast food restaurant.