This letter is called Ta-Marbúta. It is a combination of "H" and "T".
If the word is isolate or comes at the end, then it's OK to say it as "H". However, if the word has to be declined (i.e. be in nominative, accusative...etc), then we must add vowels to this ending letter. When adding vowels (we express that by saying moving the consonant) then the H sound changes to "T".
In this example above, غرفة (ghurfah) has become as nominative in the beginning of the sentence, so we should add Dhamma (u) to its end. But, the word is also indefinite (does not have AL, i.e. "the" as in English) then this (u) should be changed to Tanwin and becomes (un) in this case. Thus, the final pronunciation of the word becomes (ghurfatun) غُرْفَةٌ (and I'm not sure why Duolingo does not display the Tanwin marks).