Slide lock rings are not standard on all trombones, but they can be found on many models. They are particularly common on student-level trombones, as they can help to keep the slide from moving out of position while the student is playing.
If you are not sure whether your trombone has a slide lock ring, you can check by looking for a small, metal ring near the top of the slide. If you see a ring, it is likely a slide lock ring.
To use a slide lock ring, simply tighten the screw until the ring is snug against the slide. This will prevent the slide from moving freely, so be sure to loosen the ring before you attempt to play the trombone.
Slide lock rings can be a useful accessory for trombone players, but they are not essential. If you find that you do not need a slide lock ring, you can simply leave it off of your trombone.