This pane gives some infos about XProtect, the integrated protection system of OS X. Xprotect checks each new file downloaded from your web browser, received by email or by instant messaging to protect the system from any malware (or malicious software).
The list on the left displays the list of recognized XProtect malwares. By default, OS X regularly searching for new malware definitions (every 24 hours) and the list is updated if necessary. You can force the search by clicking on the "Update" button.
The list on the right displays a list of all downloaded items and tested by XProtect.
Note: If you select the "Remove the alert message on first opening applications downloaded from the Internet" option from the Parameters > General pane, XProtect will not check the downloaded files anymore.