A coloring blanket is a soft throw with line art printed on it, ready to be filled in with fabric markers. 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. Choosing the design is half the fun, so it is worth looking at the whole pattern before picking.
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. Mixing both on the same project is possible, as long as you remember which parts will stay.
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.
Adults often use it as a quiet, screen-free way to unwind in the evening, much like an adult coloring book. As a gift, it works for people who like crafts but already have plenty of coloring books.