The zero knot is the knot formed when the ends of a piece of rope are joined without any twists or crossings. It even looks like a zero!
When a zero is added to a number, the result is the same number. Similarly, when the zero knot is added to another knot, the result is that same knot, unchanged, except for the addition of some extra rope.
The zero knot is a knot whose crossing number is 0.