The Arbor knot is very easy to tie and is used to fasten your backing to the reel.
Wrap the line around the arbor of your spool and tie an overhand knot around the standing line. Tie a second overhand knot on the tag end a few inches from the first. Moisten the line and the two overhand knots.
read full article @ http://www.iwantdirect.co.uk/pages/fly-fishing-knots-you-will-need