In a fine-arts context, a stretcher bar is the wooden frame that canvas is pulled across (stretched onto) and attached to using staples or nails. A stretcher can be bought ready-made as four bits that fit together, or you can make it yourself if you've some basic DIY skills.
With quality stretchers, the edge facing the canvas isn't flat, but at an angle; the edge may be rounded too. This is so that when you're painting the brush doesn't hit the edge of the stretcher, creating a line in your painting. If you encounter this problem, put your free hand around the back of the canvas to hold the fabric away from the stretcher as you paint that area.
