Once you enter the title of your post or page, WordPress will assign it a “Permalink” which is basically its name.
On the left side, choose Settings, then Permalinks, and you will see:
WordPress automatically assigns the “Plain” name, which is just a number.
I prefer “Post Name” which is also the page name when you are creating a page. This makes it easy to remember the name when I am typing in the address and want to go directly to a certain page or post. It also looks nicer in links.
You can play around with the various formats, HOWEVER, whichever format you use when you create and save the post, will be the format for that page or post. If you decide on a standard format or change your format later, you will need to go into each post and page and click the Edit button to change the permalink to your new format.