Some DayZ Mods are required to run only on the Server Side of your DayZ Server. This is rare and you will probably never have to add a server mod, as almost every mod on the Steam Workshop is a client and server mod. But in the rare case you want to add a Server Side mod, here is how to do it;
If the Mod is yours, navigate to your File Manager on the Control Panel and select 'New Folder' and name the folder accordingly, for this example we will name the folder @MyServerMod
Once the @MyServerMod Folder has been created, view it's contents (which will be empty) and upload your server PBO by selecting 'Upload'
After you have successfully added your new Server Side PBO you will now need to add to your commandline so the server starts with this server side only Mod. To do so go to the 'Commandline Manager' section of your Control Panel and Select 'Custom Commandline' and select 'New' and add your description, this can be anything. Now you have a custom commandline press edit on it and add the following to the end of the commandline -servermod=@MyServerMod (Or whatever you originally named your folder) and save your edits.
Thats it, you have now successfully added a Server Side only Mod to your DayZ Server.