Essentially a one inch strap between two trees (or strong posts), tightened in a specific way with caribbeaners, so that it holds tension - and you walk on it! Or sit, jump, bounce, etc. it's like a bouncy tight rope (usually closer to the ground) There's a little hurdle to get over while you develop your stabilizing muscles, but very enjoyable once you can stay on the line! (Which takes just a few hours of practice for most)

I recommend YouTube for the knots/tightening. Everything can be bought at REI or on Amazon ( ~60' 1" tubular webbing + 4 standard climbing Caribbeaners)

If you buy some extra webbing, you can run a 'training wheels' line a few feet above your head and use it to keep yourself on the line until you can manage without.

Start low, and practice looking in front of you rather than at your feet. Good luck!