One of my website presence clients recently asked this question and it caught me off guard. 
Sometimes things we think are simple, just aren’t that obvious, and that’s okay.
A “social media bookmarking icon” is a somewhat generic term for a styled image you would put on your site to send people to the various social networks you are connected with, nothing more.
These icons can be custom made, or pulled from various “packs” that you can find around the web, like this one.
25 Free Social Media Bookmark Icons for Your Website
Simple as that!
