Round Turn and Two Half Hitches (video)
A Round Turn and Two Half Hitches is used to tie a line to a ring, very good if the line is under tension either when tying or undoing the knot.
The name of the knot describes it perfectly.
To tie the knot, loop twice around the ring and this will take the strain whilst you tie the knot. Go around the standing end to make the first half hitch and pull tight. Then create the 2nd half hitch by going around again in the same direction.