It's a method of creating pictures using repetitive patterns.
You start by making a four point box and then either drawing a simple figure in the middle or a zig-zaggy line called a "string." You are dividing the space into interesting shapes which is the basis for your various tangles. Then you start filling it in one section at a time.
There is a soothing, meditative quality to producing tangles, and you can do them anywhere. Like at your desk when you are pretending to be working or during a boring meeting (don't worry, I won't tell) and you never need to be near a plug to recharge!
It's a nice way to quiet your mind without using technology. Just grab a pad and a pen and start tangling. You don't have to be a great artist or even draw well to get some decent results. And it's FUN!!!