Check In / Check Out utility is an add-on that is installed separately. Once this utility is installed there will be a slight change in the working of the check in / check out facility available in the product.
Before installing the utility, when you check out a document, the document is downloaded to a common download folder. If the check in/check out utility is installed you can configure a check out folder in the server and the documents you check out will be downloaded to this folder.
While checking in in the normal scenario, file that you want to check in has to be selected. But if the utility is installed, while checking in you need not browse and select the file to checkin, the file is selected automatically by the system from the check out folder.
This utility will be enabled based on a settings in the Personal Settings>>Documents.