An allowance added to a curved sleeve to fit into the straighter line of the bodice armhole.
What this means is that if you measured (i) the curve of the sleeve cap, and (ii) armhole curve where the sleeve is to be fitted, the sleeve cup measurement would be larger. The extra amount is called Ease, so called because it is needed to "ease" the fabric into the armhole curve.
The amount of Ease that is required/added depends on the type of fabric used and the style you are wanting to achieve.
Generally, instructions for creating a Sleeve Block require up to 2 inches / 5 cm of ease. (Note: This is the Standard Sleeve Block that is used with a Standard Bodice Block, so we are not taking into account working with stretch fabrics).
The difference between the measurement of the sleeve cap (17.78 in) and the bodice armhole measurement (total = 16.4, back and front combined) in the Size 12 Bodice Block is 1.38 inches.
This extra allowance or "ease" makes it easier to sew the sleeve head into the armhole.