"Okay, where are you now?"
Let me break it down:
* Achcha means "okay" or "alright."
* To is a conjunction meaning "then" or "so."
* Abhi means "now."
* Kahan par ho means "where are you?"
* Aap is the formal form of "you."
Therefore, the full phrase "Achcha to abhi kahan par ho aap?" is a polite way of asking someone "Okay, where are you now?"