Side reins are two reins, usually with some give, that attach from the rings of the bit to the girth or to the rings on a surcingle. Side reins should only be attached to a snaffle bit, preferably a smooth snaffle with rings that are unlikely to be pulled through the mouth. 

Side reins should be adjusted so that both reins are of equal length. They should be adjusted loosely or left undone until the horse has warmed up, and then adjusted accordingly for the training/experience of the horse, as well as the gait they are being worked at. Ideally, the side reins should be attached to the girth or surcingle at a point equal to, or higher than the point of the shoulder.
Side reins are quite simple; if the horse tries to put its nose beyond the length allowed by the side reins, then the horse will feel pressure on the bit until they put their head back within the allowed range. The angle of the pressure depends on the position of the horse’s head, as well as how they have been attached to the girth/surcingle.

Side reins are useful to teach a horse to give to pressure. It is best to already have instilled the idea of giving to pressure prior to using the side reins with groundwork, otherwise, the horse may not know how to get away from the pressure and may end up damaging their mouth and trust in the bit