。
The post office is along the street.
Literal
Post-office [topic-は] street [along-に沿って] exist-[polite-ます].
に沿って ('along, following') is the te-form of 沿う ('to run along, follow') doing duty as a compound particle. The に marks the street as the thing being followed. Compared with the noun-suffix form 沿い (as in 通り沿い 'along the street'), the verbal 沿って is slightly more formal and prose-like.