I get asked this question pretty often, and I usually respond with “A developer who talks to developers”, but this recent interview between John Shewchuk and Business Insider explains it pretty well.
The Developer Experience team is intended to be the company’s public face among the techie set, parachuting in to help customers and partners with whatever problems they have — even if it’s a problem that doesn’t involve any Microsoft technology. It’s like a Genius Bar for the real geniuses.
The goal is to highlight how plugged-in the company is into the trends, processes, and concerns of the modern developer, even if that comes at the cost of promoting Microsoft technologies directly. Instead, they’re goodwill ambassadors, just promoting Microsoft’s mission.