Translation:You descend from the forest into the city.
Yes, silvā is ablative, because it is the object of preposition dē (which requires an ablative). Therefore, ablative silvā has a LONG a ("a" with a macron, or long mark).
However, the macron is NOT an accent mark. The word is still pronounced "SIL-va" (with the final -a "held" longer, like a quarter note compared to an eighth note).
The macron shows that a vowel is long, but does not (in and of itself) mean that the long syllable is accented.
(If a long syllable is the "second syllable from the end" of the word, however, it IS always accented. You see that in infinitives, like: currere , to run (CUR-re re), with a SHORT e in the second-to-last, or penultimate, syllable; contrast habēre , to have (ha-BAY-re), with the LONG e in the penult, which necessarily receives the accent.)
Note also that all prepositions that have the meaning FROM (namely, ē/ex, ā/ab, and dē) always require an ablative object.