If you look at the WordPress post screen (either on WordPress software or you can create free account at wordpress.com in 2 minutes) then you will see that when you set the post title, there's an option below (yellow colour) where you can set the text that
will appear in the URL. WordPress creates these by default from the title, using just alphabets-numbers & replacing space with hyphens. For example, if my post title is "Hello World" then my post URL will become:
The post slug here is "hello-world". WordPress lets you customise this so even if my post title is "Hello World" I can set the post slug to "hey-there" & the url will become:
This feature is especially useful when you write post title in languages other than english, because then the characters will not be URL encoded & make long links. So if my post title is "आज की ताज़ा खबर" then I can set the post slug to:
You can read more on this at WordPress Codex
here & here.