In order to get started working with knots, we need to understand what mathematicians mean by the term "knots". A "mathematical" knot is just slightly different from the knots we see and use every day.
First, take a piece of string or rope. Tie a knot in it. Now, glue or tape the ends together. You have created a mathematical knot.
The last step, joining the ends of the rope, is what differentiates mathematical knots from regular knots. The kinds of knots mathematicians work with are always formed on a closed loop (no loose ends).