The word is a hybrid of Zentangle and mandala. Zentangle is a type of drawing that involves the use of structured patterns, while a mandala is an intricate circular symbol from Hinduism and Buddhism that represents the universe. I just draw mine for fun and relaxation, so I'm going to use the word zendala rather than mandala.
I pretty much always have a zendala going in my sketchbook. I work on them when we're watching TV or hanging out with Ransom in the living room. Here are a few that are in my current sketchbook (which is almost full thanks to the 100 Day Project).
You should try drawing some yourself! You just start in the middle and grow from there, adding shapes and lines and dots and bumps and whatever you feel like. I rarely finish drawing one in one sitting. No rush.
Don't worry about being precise, mine certainly aren't. I've found that the more detail you add, the more it appears precise. I suppose our eyes get distracted by all the pretty shapes.
A while back I painted some zendalas all smooshed together. Then I ordered it as a laptop case from my Society6 shop (I use the case for my iPad, not a laptop. Not that that is important...). I think it turned our rather lovely.