"Estar" is often used for locations followed by the preposition "en", It could mean "to be located in". For example: Pekín está en China = Beijing is in China = Beijing is located in China

Su oficina = Subject está = verb en el centro de la ciudad = verb complement (in this case is called "complemento circunstancial de lugar"). This kind of complement is easyly recognizable because it's the equivalence to "where?" Other examples: Mi casa está en las afueras de la ciudad. La cartera está en el bolso. Las llaves estában en el coche. Espero haberte ayudado. Seguro que algún compañero te lo explicará mejor que yo. :-)

Well, I think you know that: Estar = Be En = In "Estar" is the verb "to be" and "en" is the prepotition "in or on" but you can only use it in locations or places I hope I'll have help you (I'm learning english)