Getting tracking force just right is a big part of how your turntable sounds.
Each cartridge is made with a recommended tracking force to suit the "springiness" of the needle and how it responds to music.
On Pro-Ject turntables, the counterweight scale is in units of "mN" or millinewtons (equivalent to 1/10th of a gram), so when the scale says "18" your tracking force is 1.8g.
When you balance your tonearm, you can aim for the following tracking forces for each model of Pro-Ject turntable:
Primary
OM series - 1.75g
Essential series
OM 5E - 1.75g
OM 10 - 1.5g
Debut series
OM 10 - 1.5g
2M Red - 1.8g
1Xpression series and above
2M Red or 2M Blue - 1.8g
2M Bronze or 2M Black - 1.5g
Quintet Red - 2.3g
For any other cartridge type, there's almost always a recommended spec from the manufacturer.