You probably saw them becoming more available in the second half of 2016. They started on articles and videos, then moved to questions and eventually made it out to group discussions and several other locations on the site.
That's great. What do they do?
Endorsing improves the user experience. The community endorsed solution or comment is helpful to some people. It also gives members a positive feeling when their content is endorsed. That's the most important thing.
Also, if a solution gets at least 3 endorsements it becomes a community endorsed solution.
If you're out there in the wild, and see comments that you agree with or know work, help each other out and don't be afraid to endorse each other's work! Future readers will appreciate your contributions, even if that is just a thumbs up.