Fjar file is a jar file that might contain fix / workaround for the issue or might have some debug traces which will be needed by the development team for debugging purpose.
How to apply ?
1. Download the .fjar file
2. Place it in the fixes directory under <server_home>.
For eg. If the ServiceDesk installation folder is C:\Program Files\ManageEngine\ServiceDesk ( this is the <server_home> ), then create a folder named "fixes" ( case sensitive ) under "ServiceDesk" and place the .fjar file in it.
3. Restart the application service for the classes to take in effect.