Ease is the amount of fabric that is added to a garment in addition to what is strictly required to make the garment fit.
Without ease, a garment would fit you so precisely that you might not be able to get in to it, or would have problems moving in it.
Ease that is added to allow you to move inside a garment is called wearing ease. Extra ease can be added beyond what is functionally needed. This is called design ease and is used to achieve a certain visual effect.
Ease can work the other way too. When the fabric stretches, we can choose to use less fabric than what is needed to make the garment fit. This is called negative ease.
Negative easy is used for stretchy materials where the garment must fit tightly. For example, in swimwear, tights, underwear, or sportswear.