First up, what are dynamic URLs?

They are created 'on the fly' by pulling data from somewhere on a site and building a page according to the results of the data query. They do not exist as a static page so you cannot type in a URL and pull up a page in a browser as you would normally expect.

They can be identified by having the character '?' in the URL string.

When building websites I have sometimes created them without realising it, when I have linked the category of a blog post to the blog post page. What this does is to allow the viewer to see the category assigned to a post and then click on that category and view all the other posts for that particular category. So if a website blog had a category of 'Advice' and someone clicked on a post in the category of 'Advice' and then wanted to see all the other Advice posts they could click on the category link and a page containing all the Advice posts would get dynamically built.

Dynamic URLs can also be used to pass parameters from one page to another which can be useful in some contexts.