When you're browsing a site and you like it, you bookmark it. Right?
That link gets saved into the bookmark folder of your browser, which is only accessible by you. That bookmarked site is private.
Social bookmark is exactly like that ONLY instead of your browser, you're bookmarking a site online. And it's not private but rather public, accessible by everyone.
That is social bookmarking.
There are many websites/platforms that let you bookmark your favorite sites and let others find them.
Some of the top and popular social bookmarking sites are:
1. Pinterest
2. Mix (Earlier: StumbleUpon)
3. Digg
4. Reddit
5. Scoop.it
6. Dribble
7. Slashdot
Traditional social media platforms, too, at times, are recognized as social bookmarking sites. Like Twitter, LinkedIn and even Facebook. Not that they help you bookmark your favorite pages; they just make your shared content discoverable to a lot of people.
Now each of these social bookmarking sites come with their own unique interface and features. However, the majority of them provide enough options (including tagging) to properly categorize your content/link in the right groups. This helps other people discover that content by going in the right categories.
Social bookmarking is a prevalent practice in the SEO community. Although it is now highly ineffective. Aside from the top names (Pinterest and Reddit), very few people really visit the traditional social bookmarking sites.
SEOs regularly bookmark their sites and content on these platforms so to get backlinks. That's its biggest benefit. Organic content discoverability is at its all-time low.
So, in short, social bookmarking is nothing but a vanishing practice, left with not many benefits.
Hope this answers your question.
