They sell high end Linux systems, and maintain the Ubuntu-based Pop!_OS Linux distribution. The Lemur Pro — and other Intel-only laptops — are running open source firmware developed by System76 in collaboration with Intel.
Who buys computers from System76? People that care about buying hardware that's guaranteed to work with Linux, from a company that supports Linux. That ranges from web developers to NASA.
I'm curious, but the "about us" doesn't describe what the business is trying to do, just who's doing it. I assume it's on here for a HN-relevant reason, but I'm not really seeing what it is...