SEO friendly url is the one which is a static url which makes it easy for the users as well as the search engines. You can also insert keywords in them

Elements of a URL
Subdirectories
Page Names
Static URLs
Dynamic URLs

One of the most basic search engine optimization techniques is in using SEO-friendly URL structures.( use short url and use keyword in your url it will help you in SERP)

Poor URL structure is a frequent SEO issue, one that can impair rankings, keep pages out of the search engine indexes, and suck ranking authority from your other pages or even the entire websites. ... Others generate multiple URLs for pages, creating duplicate content.

Poor url structure is a common seo difficulty, one that could impair ratings, keep pages out of the quest engine indexes, and suck ranking authority from your other pages or maybe the complete websites....Others generate multiple urls for pages, creating duplicate content material.

SEO Friendly URL is keyword based, have short length and usually your keyword with dashes replaced.

Poor URL structure is a frequent SEO issue, one that can impair rankings, keep pages out of the search engine indexes, and suck ranking authority from your other pages or even the entire websites.