Translation:The girl could not eat a whole loaf of bread.
Welcome to the quirks of the Danish language! There is no word for loaf.
I don't know if you could use "loaf" by itself like that in English though, it seems kinda wrong.
But in Danish et brød is considered "a loaf of bread", so de spiser et brød → they eat a loaf of bread.
De spiser brød → They eat (pieces of) bread (doesn't quantify how much so they could be eating anywhere between 1 and 500 or whatever).
De spiser et stykke brød → They eat one piece of bread.