A deposition is a statement taken under oath during the discovery phase of a lawsuit. Usually, an attorney representing an opposing party will ask questions about a witness' background and about facts related to the lawsuit.
If the witness has an attorney, the witness' attorney should prepare the witness by explaining the deposition ground rules and by representing the witness at the deposition.
A court reporter attends the deposition and takes verbatim notes, which then become a transcript for use at trial.