Ease is how much larger the garment is designed to be than the body wearing it. It is added to the shopper's measurement before comparing against your size chart, so it decides which size wins.
How ease is entered
Ease is entered in centimetres. Leaving it blank uses a default from the product category, based on your size chart:
Men's long-sleeve shirt: 10 cm chest room
Blazer: 12 cm
Jacket: 17 cm
Leggings and wetsuits: 0 cm
When to change it
If the size recommendation is consistently one size too large or too small in the same direction, ease is the setting to adjust. Roughly, moving ease by one size-gap moves the recommendation by one size.
On the Advanced recommender
Ease also drives the fit preference on the Advanced size recommender. Each of the seven fit steps is a multiplier on the ease value you have set — so if you set ease to zero, the fit slider stops doing anything at all.
Setting a custom ease value by hand replaces those multipliers with your flat value. This is why the fit preference question disappears when you set a custom ease on any measurement.
On the Basic recommender
None of the above applies. Ease is used as a flat value and the fit preference works independently alongside it.