This is the 2099th most frequent French word.
anciennes
"Anciennes" means "old" or "former" (feminine plural).
Here, "anciennes" is used as an adjective to describe "clés" (keys), indicating that they are from a past time or pertaining to a former condition.
J'ai retrouvé mes anciennes clés dans un tiroir.
I found my old keys in a drawer.
In this case, "ancienne" is used as an adjective describing "collègue" (colleague), referring to someone who used to hold that position relative to the speaker.
Elle est une ancienne collègue de travail.
She is a former work colleague.
"Anciennes" here describes "rues" (streets), emphasizing their historical aspect or their longstanding presence.