Some adjectives can go either in front or behind the noun it modifies, with a possible difference in meaning.
With "les longues robes", "longues" would be a subjective size attribute (conforming to BANGS). "Les robes longues", on the other hand, refers full-length dresses -- a distinct category of dresses. They are not just long versions of, say, summer dresses, so in this case the adjective is not a size attribute.
Here we are translating from the French, and its word order tells us that we are talking about a category of dresses, although the distinction is lost in the English translation. In the corresponding exercise in the reverse translation, I think both word orders in French are accepted, because the ambiguity exists in English. Outside of a short translation exercise, the context can help to narrow down the intended meaning.
Since robe is feminine, singular would be "la robe longue". Even for masculine nouns, the sounds of the articles "le" [lə] and "les" [le] would be clearly differentiable. (Hear both on http://www.larousse.fr/dictionnaires/francais-anglais/le/657898. For help with IPA vowels, see https://en.wikipedia.org/wiki/IPA_vowel_chart_with_audio.)
It is a tool used to apply some rules that require a fair amount of judgement. Most of the adjectives that fall into the BAGS categories qualify to be placed in front of the noun most of the time. Less need to make decisions about what is and what is not an inherent quality or whether an adjective can be said to classify the noun.
(Inherent qualities go in front, descriptors that classify go after)
When you get comfortable enough with the BAGS rule to wish for a more complete approach consider this:
Subjective/figurative adjectives go in front. Objective/literal adjectives go after. Some adjectives can be placed in front or after depending on your intended meaning. All adjectives are subjective to some degree.
When you get comfortable enough with those rules and want an even better understanding, you can go to about.com where they have a couple of pages of detailed procedures for adjective placement.
However, at your current level in Duo, BAGS will handle what you need.
The other thing is that most of the time, except when you are being tested such as on Duo, it isn't exactly a big mistake, as mistakes go, to place an adjective inappropriately. Often, its just a small distraction. However, if you wish to say Hitler was un grand homme (a tall man) and instead say un homme grand ( a great man) you may get some odd looks. So it can matter.