The sentence says, "They are at lunch," not "They are at the lunch." Knowing this information, let's look at this: a=at (or to) il=the (if it was a feminine word like "cinema" then it would be "la", not "il")
now we combine the two words to create "al", so: a+il=al now we can assume: a=at il=the al=at the
So if you understand that, then you'll understand this: They are AT lunch=loro sono A pranzo (a=to)
So the word "al" would be if the sentence was: They are AT THE lunch=loro sono AL pranzo
I hope this wasn't confusing. Good luck :D
It is a difficult question. In the English course for Italian (by Duolinguo), the students bickered about a similar phrase... What is correct "andare A lavoro" or "andare AL lavoro"? Well, the correct answer is: "andare AL lavoro"... in this case there is no real "grammatical" reason.
In general we can say that the difference is the presence or absence of "definite article" in the "preposition"... so we always say "Loro sono A pranzo"... instead we say "Loro sono AL pranzo di nozze"... In this case we speak about a specific "pranzo" and in Italian we need the "preposition combined with a definite article"... A + IL = AL
Normally you will find:
Andarea teatro... but "Andare al Teatro dell'Opera di Roma"
Andare a scuola... but "Andare alla* scuola elementare"
Andare al mare
Andare al cinema
Andare al bar
Andare all’edicola (o in edicola)