Easing distributes a slightly longer edge into a shorter one without making folds, most commonly when setting a sleeve into an armhole.
How much extra length there is sits in the pattern. On a digitized piece you can measure the sleeve cap against the armhole and see the difference before sewing.

Digitize sewing patterns without tracing.
Test PatternScan Pro free for 14 days with the full feature set. You need your smartphone, a printer and one pattern you always wanted digital.