A drawstring is a loop of string or cord that is sewn into a seam that circles around an opening, so that when the ends of the cord are pulled, the loop shrinks, thus closing the opening.
Drawstrings are common at the mouths of cloth bags (from purses to sleeping bags), the tops of loose pants (such as pajama bottoms), and the bottom of jackets, and hoods, among many other things.
Drawstrings are simple to make, often hard to fasten tightly, and when fastened by tying, potentially hard to unknot later. There is also the risk of losing one end of the string into the depths of the seam, never to be seen again. Some very cheap goods are made with drawstrings (minimalist backpacks, in which the drawstring doubles as the shoulder straps, and pajama bottoms). Nonetheless, drawstrings can be quite classy. Small drawstring bags can be very cool (particularly when made from a fancier material, like leather or silk), many very expensive purses are theoretically drawstring bags, and pajama bottoms are 'in' just at the moment.
Because everything must be noded. EVERY LITTLE THING.