Let's say you are a webdeveloper trying to package your js app into a executable.
Tauri makes it so that it embeds your js into a rust base, and if you need to interact with the os, you have a Tauri JS API which calls predefined rust functions embedded in the executable.
You can literally never write rust code while using tauri.
But they givr you the choice, since rust is faster than compiled js, you also can create rust functions which you then call from your js code.
Seafile has fuse for accessing it as a folder.