The Mods in the list to the left are all installed on the game server. You can not play on the server without having all of the Required mods installed. If you have mods that are not in the list installed, you will not be able to join the server.
There are links to download individual mods, if that is all you need, in the list to the left. There are also zip files containing all of the required and optional mod files in the Mod Package Downloads link in the main menu to the left and Mod Manager Profile Codes in another link in the main menu.
* VERY IMPORTANT * If you have existing mods installed, be sure to delete all of the .dll files in your Valheim\BepinEx\plugins folder. If you have any .dll files in your plugins folder that are not in these these .zip files, or if you don't have all of them, you won't be able to connect to the game server.