Deposition is simply a process in which either the plaintiff or defendant of a lawsuit can ask questions to the other party or a witness about facts relating to that case. It is usually done at the office of the party requesting the deposition in front of a certified court reporter and possible an interpreter if the deponent requests one. If you received such notice, you would have known about it through your attorney and he can help you. If you are representing yourself, you need to prepare to answer any and all questions regarding the case. There may also be a request to bring certain documents at the deposition.
If you are still unsure, you need to consult an attorney.