It looks like an ordinary throw, except the design is printed as outlines that you colour in yourself. Most measure around 150 x 125 cm and come with a set of pens in the box.
You pick a section, colour it in and come back to it another evening. Several people can colour at once, so it suits couples, friends or a quiet evening round the table.
White lines on black make every colour stand out, even the lighter ones. Choosing the design is half the fun, so it is worth looking at the whole pattern first.
What makes the difference is the washable ink. Washable pens use water-based ink that, per the manufacturer, washes out in the machine, so you can start again. Check the care label before the first wash.
Permanent fabric pens are for the reverse: a design you want to keep, which will not wash out. It helps to decide before you start which parts should be permanent.
Some simple habits make it easier. Lay the blanket flat on a table rather than on your knees, and put the caps back on the pens between sessions. Testing a colour on a small corner shows how it looks on the fabric.
For adults it is a calm, screen-free way to unwind in the evening, much like a mindful colouring book. As a gift, it suits people who love crafts but already own plenty of colouring books.