Discovery is the evidence related to the charges in your case and in the possession of the government.  The material can be statements of witnesses, documents related to the crime, investigative agency reports, criminal history, forensic evidence such as fingerprints, DNA, photographs or any number of other types of collected information such as surveillance tapes, or telephone records the government has obtained by subpoena power.  One of the first things counsel will do is seek to obtain this material through legal procedure mandating such disclosure.