Hi and welcome here, lemon13!
Joplin stores the data on your device, so the data is always as safe as your device is safe. Everyone, who has access to your system can open Joplin and see every note. So if you share you system with others, you can't make it safe easily, as Joplin does not support access limitations by itself.
Over all it's a well discussed topic here wether to encrypt the Joplin database on the device or not. But at present, it is not planned to implement that. If you like, have a look here for the arguments.