Quill is a simple Micropub client for creating posts on your own website. To use it, your website will need to have a Micropub endpoint, and this app will send requests to it to create posts.
There are Micropub plugins for various content management systems such as Wordpress, and is supported natively by some software such as Known. It's also a relatively simple protocol you can implement if you are building your own website.
Once you've signed in, you'll be able to use the various interfaces see an interface like the one shown which you can use to write a post. Clicking "post" will make a Micropub request to your endpoint.