It’s called a Kikkerland Rhino II Folding Stool and they sell for about $11 on Amazon with free shipping. The version II is sturdier than the original design so be sure to get the Rhino II model.
When folded, it measures about 12”x 12” x 1", so it fits nicely in my backpack. I mostly use it for macro photography. For me, this means either wildflowers growing in nature or cultivated flowers growing in botanic gardens.
Flowers, by their design, tend to lie low to the ground, and this stool really helps me get down to the level of the flowers without resorted to sitting crosslegged on the (often wet) ground, or getting on my knees. My joints feel a lot less “creaky” at the end of a session. It makes the process a lot more enjoyable, whether you use a tripod or, like me, avoid a tripod whenever possible. I find the stool helps me hold the camera steady. Often I will put my elbows on my thighs, creating a “human tripod”.
When in the field, once I have taken the stool out of my backpack, I generally don’t put it back. When folded, It’s so easy to carry by the handle.
When not used for my photography, I usually just keep it in my car. It takes up so little space. We now have a second one in the house. It comes in handy for changing lightbulbs in the ceiling or swapping out batteries in the smoke detectors, and then folds up nicely to be hidden in a closet (or, in our case, between the washer and dryer).
Hey, it's only $11. Buy one!