Essentially, it is a framework where Gartner maps out all the latest technologies against the current hype and their level of adoption. It's famous in the ICT space - we referred to it all the time when talking to our clients at Internet Solutions - and I suspect it would bevery useful for marketers as we increasingly take CIO budgets into CMO budgets.
The idea behind it is that technologies usually take a while to become useful, or as Gartner puts it, "productive". As agencies, we want to be riding the wave up the "Slope of Enlightenment" in delivering services and technology for our clients. As marketers, this is a nice way to map our strategies for customer touch points.
Source: Gartner, 2014 - Digital Marketing.