It's not too difficult. Let's take it apart.
"Zitten" means "to sit," but it could also be used as "to be." Like in English "I'm sitting right here." and "I'm right here." pretty much mean the same thing.
So "Julli zitten" is "you are."
Let's just ignore "het." A lot of European languages add a direct article before the name of a meal, but we don't do that in English, so drop it.
"Aan" is "on" "Ontbijt" is "breakfast"
So we have "You are on breakfast." This sounds weird in English, but if you're a native speaker, you can figure out what it means.
It means: "You are at breakfast" (this is the translation I went with, and it was accepted) "You are eating breakfast" "You are having breakfast"
The word "having" in the correct translation is there simply to make the English sound natural.
Hope it helps.
I think "You are at breakfast." is more of a literal translation. It can be said, but is not as common as the true translations: "You are having breakfast." or, what I put and got right, "You're sitting down to breakfast." (which looks more like the Dutch construct) As for coming to the answer given by Duo, the true translation here is relatively easy to get since the words in Engels are almost the same as those in Nederlands (you-sit-at-the-breakfast). Just remember to translate idea-for-idea and not word-for-word.
I will add something that my Dutch teacher explained to me. He said that in Dutch, they often like to use a positional verb instead of just "to be"; meaning that "I am downstairs", and a Dutch person might say "Ik sta beneden." Stand, sit, lie -- if you look, you'll see those verbs in idiomatic Dutch sentences, where we would translate them in english to simply "to be". All that said, it's tricky to know when to use them, and when not to. Just like 'het' and 'de'.
This is what I understand from my investigations: You can say "zijn(to be conjugated) aan het + verb" and it will mean it's continuous. For example: ik ben aan het eten = I am eating (instead of I eat)
And you can also replace the verb "zijn" with "zitten" and it will mean the same thing, but it will have the implication of sitting, such as sitting to eat lunch or breakfast.
you could say "jullie eten het ontbijt", but I think saying "jullie hebben het ontbijt" is more like saying you posses breakfast and not necessarily eating it. Saying "jullie zitten aan het ontbijt" is pretty normal i would believe. In Dutch when stating where something is or you are doing something you would also say how it is positioned as well, "het boek sta op de plank" is "the book is on the shelf". As for "het ontbijt" this is the same as when in English we must include the article to accompany the noun e.g. "the sea and the sky". In Dutch it isn't always necessary to include the article when talking about certain things e.g. "de borden liggen op tafel" becomes "The plates are on the table".
Hope this helps, I feel like I have done a terrible job explaining this, but I learned this years ago from a book that I no longer have.
Feel free to correct the hell out of this native speakers.
I thought I figured it out, that "zitten" is "am/are sitting", not "to sit." But I've just tried "You are sitting at the breakfast," and now it has a problem with the "the". I like to know the literal translation of the words as well as the idiomatic meaning. Is the het in this sentence not a "the"?