I hadn't know what they did either, but their homepage makes it abundantly clear: https://buffer.com/

They let companies and individuals manage & schedule social media posts across a few of the main platforms (At the risk of doing them a disservice in my explanation, but also saving whoever reads this a click).

I had the same question. I clicked on a few links that I thought would lead to the product description but they didn't. Eventually I realized that the domain was open.buffer.com. Once you go to buffer.com, you see the product description. From the homepage:

"The best way to drive traffic, increase fan engagement and save time on social media."

Social media marketing automation