It is absolutely essential that you know this knot as it forms a loop in the end of a line that will not slip. It is therefore a life saver. If you ever have to tie it snugly under the armpits of someone who has fallen into the water from a boat then you will quickly realise - too late - that it takes practice to tie it when you are looking at it from the wrong direction. Spend time NOW learning to tie it upside down.
First form a loop in the standing part of the line. Then pass the working end (around the distressed soul and) up through the eye of the loop, around the back of the standing part and then back down through the eye once more. See figures 1 & 2. This forms a simple and secure knot which holds tight the greater the strain placed upon it but can always be untied easily by pushing forward the bight which encircles the standing part.
Practice it until you can tie it with your eyes closed!
Figure 3 shows a less secure way of tying it which should be avoided. The end of the working part of the line should pass into the loop rather than out of it.