From Marion Boddy-Evans,
Your Guide to Painting.
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Have you ever wondered what those little bits of wood that come with a stretched canvas are for? Sometimes they’re stapled to the back in a little bag, sometimes they’re already wedged into the mitred corners of the stretcher bars (the wooden frame the canvas is attached to). That said, some cheaper canvases I’ve bought have plastic ones.

These bits of wood are canvas tightening keys or wedges, used if the canvas begins to sag a little on the stretcher. You put them into the slots made for them in the corners of the stretcher bars, then tap them in further to tighten or square up (align) the canvas.

If you’ve bought an unprimed ready-made canvas, or are stretching your own, don’t use keys until after you’ve primed the canvas, as priming makes the it more rigid.

Some canvases are made with rigid corners, without the option of hammering in a wedge or key. If you’re using one, just check the canvas is very taut before you buy it; it should stay that way.