What are the random characters in some post URLs?

Some older posts have an ID at the end of their address, for example my-post-title-cl26c8xdq235511jo6. It is the post's unique ID. New posts get a slug made from their title, without an ID.

The ID does not hurt SEO, and you do not need to change it.

If you want to remove it, change the slug and add a redirect from the old address. Search engines and existing links then follow the redirect to the new one.