Automatic Progress Saving
With default settings enabled, and while your server is running, your server will automatically save progress every 20 minutes.
If you stop (not kill) your server in the server control panel, your server will also attempt to automatically save the world's progress.
Manual Progress Saving
If you use uMod, you can use the AutoSave plugin to have a the server save more often. This is only available for Valheim servers with uMod installed.