It is a packet of paperwork the State sends in response to a notice to participate in/demand for discovery. It will include police reports and affidavits, but is often not the complete discovery.
It simply means that the prosecution has let the court know it has complied with the defense attorney's request for discovery , Generally speaking, discovery is comprised of all documents, videos, pictures, interviews, or physical evidence gathered in the prosecution of the case. The defense, should it so choose, is entitled to participate in the discovery process and thereby obtain a copy of those documents, videos, etc. Discovery is limited to a certain extent: for example, the notes a prosecutor writes on his or her file are not discoverable. In some states, like Georgia, defense attorneys are somewhat limited in what types of evidence they are entitled to through discovery.
"Discovery" is a legal term of art describing the exchange of information between the parties in a given case. In the criminal context (and in Florida), Rule 3.220 (which can be found at http://www.floridabar.org/TFB/TFBResources.nsf/0/BDFE1551AD291A3F85256B29004BF892/$FILE/Criminal.pdf ) governs discovery. A "discovery exhibit" is a submission, by one party to the other, of items which are discoverable.