This is a variation of the bowline and is used for joining two lines of approximately equal size together. Use it for making up temporary long lines or to quickly mend a snapped line. In either case the long term solution is to obtain a new piece of rope.
Form a bight in the end of the thicker of the two lines. Pass the working end of the other line up through the eye so formed, around the back and trap it in place under its own standing part. A second turn can be taken around the neck of the bight for s slightly stronger purchase. Note that both short ends finish up on the same side.
A quick-release draw loop can be incorporated if required but the knot, despite being strong, is easily undone by rolling forward the bight encircling the single line.
If the short ends finish up on opposite sides this is a left-handed sheet bend, not so strong and should be avoided.