It means that the speaker's office is farther from the train station than the office of the person being spoken to. If the reverse were true, the speaker might say "My office is closer to the station than yours is," or "Your office is farther from the train station than mine is".

"Yours" is a possessive pronoun == " le vôtre/le tien/la tienne" "Yours" is replacing "your office". We could also have written: "My office is farther away from the station than your office is. Il se trouve de plus loin que le vôtre. There is also some omission at the end of the sentence because the information is understood from the context. An extra specific version of this sentence could read: "My office is farther away from the station than your office is from the station." But this would be too wordy.

hi, it means - your office is. when we omit the object (office) we should transform 'your' into 'yours' for example: it is my pen, not your pen = it is my pen, not yours (object is omitted, so we have YOURS)