You can write private notes within WordPress for your own personal use. You may use it for keeping a reference to something, some text, or a link on the web to use it in the future while writing your post.
The new WordPress plugin called Hypernotes offers you a new custom post type that is completely private and created specifically for writing notes. These notes can’t be published online anyhow so it gives you a complete sense of security that it will not go online to the public even by mistake.
Otherwise, the private post option of WordPress is also available that simply allows you to make any post private (using the ‘Publish’ meta box) i.e. available only to logged-in website users.
The plugin also offers you the option to create folders and organize your notes. Together with the new PWA capabilities introduced by the Gutenberg plugin, these notes could be accessed easily through WordPress app on your smartphone.
Although the notes you create with this plugin are kept hidden from the public but at the moment these are not encrypted, so anyone having the access to MySQL database on your website will be able to read them (only if he knows how to).
But no worries if you are the complete owner of your WordPress site and use cPanel, VPS, dedicated server, or Managed WordPress for running your website because then only you have access to your website’s database and you can decide whom you want to share that access with.