A coloring blanket looks like a regular throw, except the design is printed as outlines that you color in yourself. 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. Several people can color at once, which makes it an easy activity for couples, friends or a family table.
Designs vary a lot: dense floral patterns for long evenings, bigger shapes for kids or beginners. 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. Check the care label before the first wash.
Some people switch to permanent fabric markers for a finished blanket they want to keep as it is. Just keep track of which areas were done with which ink.
Two small tips before you start. Lay the blanket flat on a table rather than on your knees, and put the caps back on the markers between sessions. Start with lighter colors; darker ones are easier to add on top than to take away.
Many adults treat it as a relaxing hobby, something to do with their hands while talking or listening to music. It is also a common gift idea for the holidays, since it is useful long after the coloring is done.