You might have just been on their blog. If you go to buffer.com you'll see they are a social media management tool
I think they are similar to Publer [https://superpubler.com], and allow you to automate your social media postings and schedule them in future
They provide a tool set for scheduling marketing content across multiple social media services.
Originally, it was built just to schedule tweets in the future.
Also worth noting that they are highly popular among marketers and community managers. It does not say anything about "what they do", but I guess it says something about how well they do it :)