It's butter that's made from cream which has been inoculated with cultures, which are microorganisms present in fermented dairy products (e.g., yogurt or creme fraiche). It's a bit tangy and tastes spectacular, so it's most popularly used as a "finishing" butter, like on toast or waffles.

The process for making it is simple, but does take some patience. I've used this video as guidance before.

This wasn't in VT, but on a hike up Mt. Garfield a few years ago I passed a guy who said to me, out of the blue, "whoever said 'leave no stone unturned' never visited New Hampshire"