This is the first knot I teach my students, it is called the Sheet Bend, which is actually a bend and not a knot, but that's splitting hairs. Knowing this knot will allow you to join two pieces of rope - cordage of the same or different diameters, forms a strong joint, and is easy to get undone. If you only learn to tie one knot, this is the one that will prove most useful to you. There are thousands of knots, but the sheet bend is in my opinion the King of all knots. I have found that most people have no clue how to join two ropes together, so get busy practicing. You should be able to tie at least 10 knots for general purposes, and be able to tie them in the dark by feel, and with one hand.