It's a formal way of saying goodbye, similar to "farewell" in English. You can use it in most situations, but it's especially appropriate when ending a formal conversation or meeting.
Here's a breakdown of its meaning:
* До (do) - "to"
* свидания (svidaniya) - "meeting"
So, literally, "dosvidaniya" means "to the meeting."
However, it's not used literally, but as a polite way of saying "until we meet again."