Discovery is the process where parties obtain information from each other before trial. This may be documents, or answers to specific questions. Essentially, "Amended" Discovery means that they are changing, or supplementing, the answers or documents that they previously provided.
Discovery is all the evidence the state will use against you in a criminal trial. If the state is has amended discovery, just as the other lawyer said, there is some kind of change in evidence, they are correcting an error in the evidence or there is additional evidence.
Discovery is required in all criminal case and is the process of the the State Attorneys office providing information they plan on utilizing should the case proceed to trial. Discovery usually includes items such as the arrest report, witness list, witness statements and criminal history of the defendant. There is usually an initial discovery exhibit provided after the arraignment. However, if any new information is received by the State Attorneys office regarding the case or they need to make a change in the initial discovery, they advise the defense of such through an amended discovery exhibit.