We often have to tell our clients that much of the actual work that we have to do in a civil or criminal action involves the drafting/serving, and response to, what the law calls "discovery." "Discovery" is a general term used to describe the process whereby lawyers exchange information - mostly long before trial - so that the parties can either work out a resolution or get ready for trial. The general concept is that there should be nothing hidden from the resolution process and no surprises at trial. There are a number of rules in both the civil and criminal justice systems that spell out how and when this discovery stage proceeds. In a civil case, the lawyers can serve and respond to written questions (called "interrogatories") or serve and respond to document or evidence requests (called "request for production of documents"). The lawyers can conduct "depositions" of persons with knowledge (who may or not be witnesses at trial). In a criminal case, there are no "interrogatories," but the lawyers do have to identify witnesses and documents (and produce documents). Sometimes (very rarely) the criminal judge can order a deposition of a sick or difficult to secure witness - and that deposition transcript takes the place of the live testimony at the criminal trial.
Lots of times the clients have no idea of the amount of work that goes into the discovery phase of a case. They see what happens when they are in court, but not what happens out of their view or hearing. On the average, a lawyer spends 3 times the amount of work on the discovery part of a case than the lawyer spends on an actual trial or courtroom appearance.
So when a lawyer starts using the term "discovery," you can assume that the lawyer is talking about this part of the case process. Feel free to ask what discovery has produced for your case - and assume that you, as the client, will have to submit information, documents, and, often, deposition testimony, as part of the discovery process.