The albright knot is typically used to tie the backing/line to the fly line. It is also popular with saltwater anglers wishing to join different diameters of monofilament, such as 50-pound test and a 15-pound tippet.
Illustrations courtesy "Fishing Central California" (Milne/No-Nonsense Guides)
First, make a loop in the thicker line (in this case, the fly line), and thread the thinner line (backing) through the loop and slowly wrap it around the fly line 10 times.
Next, after you’ve wrapped the backing around the fly line 10 times, pull the end of the backing through the loop the same way it entered. Wet the line and pull the knot together tightly. If you’ve tied it correctly, this should be a nice, tight, sturdy connection between the backing and fly line. Trim the ends of the two lines to create a smooth connection that won’t hang up on the reel.