。
The post office is in the center of town.
Literal
Post-office [topic-は] town-of central-part [at-に] exist.
中心部 ('central part, city center') is a Sino-Japanese compound: 中 (middle) + 心 (heart) + 部 (part). The ~部 suffix is productive for body-parts, subdivisions, and regions. The sentence is a neutral locative statement, typical of a guidebook.