The sheet bend knot is a simple knot which is used to tie two ropes of unequal thickness. This knot should be used instead of the figure eight or/and square/reef knot when the cords or ropes are of a differing diameter. As suggested by the name, the thicker rope could be a sheet. This is an easy knot to tie and an easy knot to check. When this knot is not under load, it can work loose. Consider tying a safety knot, to attach the working end of the thinner cord to the thicker cord.
See here for knot terminology.