Ease is the difference between your body measurement and the garment measurement at the same point.
For example, you may have a waist measurement of 72cm, but the measurement of the waist on your pattern is 80cm. This means there is 8cm ease at this point on the pattern.
Ease is there for two reasons. The first is for fit - you require a certain amount of ease so that you can sit down, bend over, move your arms (and eat) in your clothes.
The second reason is for design purposes. You may want a baggy, over-sized silhouette, so in that case the waist measurement on your pattern, may be drastically different to your actual waist measurement.