Half hitch knots are the foundation of most friendship bracelets. Double half hitch knots are one of the most common knots used in macrame. Both the half hitch and double half hitch knots are formed by knotting one strand around another. Be careful to keep the strand that's being knotted around taught - if you pull the threads tight unevenly the knot won't lay properly.
Why is it called a "double half hitch knot" instead of just a "hitch knot?" Because a "hitch knot" is an entirely different kind of knot that was probably named first!