A daisy chain is a series of slipknots tied together in series, using the loop of the previous slipknot as the base for tying the next slipknot. To make a daisy chain, start with a slipknot. Take the working side of your rope and push it through the loop of your slipknot without putting the end through so you form a new loop. Pull the knot tight cutting off the new loop. Repeat the process ad infinitum or at least until you run out of rope.
For those of us who have busy hands, daisy chains are a great way to keep yourself occupied: you can tie them out of anything from electrical cords, to shoelaces, to a stray thread and, with a little practice, you can do them without even looking at what you're doing. It can also be a somewhat useful knot. I personally tie them into my headphone wire to shorten them. If the cord gets caught by anything, the knot will just pull apart gently instead of violently yanking out the plug since it's just a series of slipknots. Still, my favorite thing to do with daisy chains are races: you and a partner start at opposite ends of a rope and start chaining as fast as you can. The person with the longest daisy chain when you run out of unknotted rope is the winner.