Just as a partitive article in French → des → is the plural of the indefinite articles un / une, dei is a partitive article in Italian and is like the plural of un/una (and, in both languages, des / dei vaguely translates to "some"). Even though the partitive is always required in the Italian and French sentences:
"Lui mangia dei panini" and "Il mange des sandwichs"
it is not required in the English translations: "He eats sandwiches."
I spoke to my Italian friend and she said that the word 'some' is 'alcuni' in Italian and that it should really have been 'Il ragazzo mangia alcuni panini'. And we have been taught that 'dei' equals 'of', so is quite confusing.