Originally Posted by Nesster
That's just a reminder of the kind of film you have in the camera. Later cameras had a little pocket in the back to hold the flap of the film carton for that purpose.
Well, also, it affects what the meter shows. If you have film that's one stop slower than the meter expects, putting the needle in the middle will underexpose by one stop.
Originally Posted by Stevopedia
Using the K1000 is really easy once you get the hang of it. Just get a combination of shutter speed and aperture that places the needle as close to the center of the gap on the right side of the viewfinder as you can.
The K1000D's meter is basically of all the light in the frame -- not even center-weighted, let alone any sort of fancy evaluation. I actually like that, once I got the hang of it. Just make sure to put the needle above or under the center if your subject is significantly brighter or darker than the average of the scene.
Of course, the meter will only work if the battery is good. To check the battery, set the film speed to 100 and the shutter to B. The needle should "peg" at the top of the meter (+) and stay there. If it doesn't, then you need to change the battery.
Random tip #2: the meter runs and discharges the battery whenever there's light. Save battery by not forgetting the lens cap.