"Les filles ont des robes."
Translation:The girls have dresses.
An s and the end of the word isnt pronounced unless it forms a liasion with the next word. Liasons are explained really well here: https://www.duolingo.com/comment/2004908/French-liaisons-between-words Hope you find that helpful. I think you could have a liasion here which would help to make the difference more obvious but the computer voice doesnt do it.
As i understood it, since you cant really hear the plural form of words when spoken, they use des to let the listener know it is plural. It literally means some; so in this case: the girls have some dresses, but in english, the word some isnt necessary since you know dresses is plural. Hope this helped!
Des is in place of 'some' (but not a literal translation). Whenever you say 'The girls have dresses' or 'we are children' or something without a number to state how many, you add 'des' instead of 'les' ('the'), and it has the same affect as saying 'The girls have some dresses' or 'We are some children'.