If you just go to layer.com, the first text you see on the page does a pretty good job of spelling out what it is. At least, it did for me. It's also more up-to-date than that comment, it would seem.
Blake from Layer here. Have you taken a look at our developer documentation on developer.layer.com? I felt like we did a pretty good job of presenting the product capabilities. Our homepage and the developer documentation speak to different audiences. Let us know how the developer side matches up to your expectations.