The Prusik knot is a great versatile outdoor knot, boating knot, or paddling knot, and is used widely throughout each. Mountain climbing is a big user of this knot. You can use this knot to secure a loop to a tight line. Also great for hitching boats.
A Prusik Knot forms a sling / loop that jams solidly on a tight line, but slides when not weighted. Mountaineers use this knot to form footholds to help them climb a vertical rope. The loop needs to be made in rope or cord that is at most half the diameter of the main line.
There are 3 basic steps to tying the Prusik. Check them out. Learn.