It's exactly how it sounds. It's a subfield of computer science dealing in pattern recognition and computational learning theory in artificial intelligence.
In Silicon Valley, people hail this subfield as the leading frontier for innovation that is applicable in almost every company. Machine Learning engineers are very high in demand as the work they produce allows for products to adapt and compute at higher levels than humans can.
Jack: The best jeopordy player and the best chess player in the world are computers.
James: That's the power of machine learning, being able to adapt to situations and read more steps ahead than a human can.
Jack: Yeah but I bet he can't FEEL like I can.
James: How do you feel?
Jack: I feel like shit actually...