The anti-skating weight is a small metal cylinder secured by a piece of fishing line. The small loop (on the right hand side in the image above) attaches to a pin on the back of the tonearm (see below), and is supported by a second pin or a wire support arm.
Once it's looped on, the anti-skating weight stops the tonearm from drifting back towards the outer edge of your record, helping make sure that the cartridge sits in the middle of the groove so you get an accurate, balanced sound in both channels.
You can see in the image below that the anti-skating pin has three different rings on it for the weight to hang from. The ring you choose affects the amount of force the weight applies to the tonearm — closer to the tonearm (1) offers less, while ring 3 offers maximum force.
Choosing the right ring has to do with the cartridge fitted to the tonearm, and how much downforce that requires. The Ortofon cartridges that come fitted on most Pro-Ject turntables need between 1.5 and 1.8 grams of downforce, so the most common ring used is ring 2. These are only recommendations, though, so we suggest trying a higher setting if you find your tonearm drifts outwards and see what works for you (make sure you've got your tracking force set properly first, though!).
The anti-skating weight is suitable for almost all Pro-Ject turntables.