Think of a coloring book, but made of fabric and big enough to cover your lap on the couch. The usual size is close to 59 x 50 inches, and the markers are often included.
You pick a section, color it, and come back to it another evening. Because the blanket is wide, two or three people can work on it at the same time.
Many designs are printed as white outlines on a black background, which gives a stained-glass look once colored. Large open areas go faster; small detailed zones take longer but look striking when finished.
The part people ask about most is washing. Washable markers use water-based ink that, per the manufacturer, washes out in the machine, so you can start over. The care label remains the reference for temperature and drying.
Permanent fabric markers do the reverse: they are meant for a design you want to keep, and they do not wash out. Mixing both on the same project is possible, as long as you remember which parts will stay.
Some simple habits make it easier. Work on a table with the fabric pulled flat, and cap each marker as soon as you are done with it. A quick test on an edge shows how strong each color looks on the fabric.
Many adults treat it as a relaxing hobby, something to do with their hands while talking or listening to music. Once colored, it is still a cozy throw for the couch.