When tying knots, being tidy and precise helps to eliminate failure under load. When we "Dress" a knot, we are striving to make the knot very recognizable and leaving just the right amount of tail ends. If you leave your tail/s too short, the knot may slip and come undone. Dressing the knot also includes removing the slack in the knot. To "set" a knot, we simply tighten the knot by pulling on all ends. This forms a good quality control check before you really need to rely on that knot. Take some care in tightening the knot to maintain its proper form. Sometimes it advantageous to leave a little extra running end after the knot (tail). You can then tie a "Backing Knot" like a simple over-hand knot after your primary knot. If the primary knot slips a bit, the backing knot will often prevent complete failure.
Example of Backing KnotsHere is a good example of backing knots: