A test strip is part of darkroom photography, the good old fashioned kind with open chemicals and safelights and manual enlargers. When you are making a print, you set it all up in focus etc, and then you put a little strip of the light sensitive paper underneath the enlarger. You flash it with 4 or 5 seconds of light, covering most of it with a piece of cardboard; then you move the cardboard over and expose more of it and then again and again. So at the end your test strip has several different exposure times on its length, then you can see what time is best to expose the full picture. The paper is fairly expensive and you dont want to waste it.