Ease is how loosely or tightly a sweater fits. The same size as the bust or chest measurement is zero ease, a little smaller is negative ease, larger is positive ease. Most women's sweaters fit well if they're from 2" negative ease to 2" of positive, though a cardigan may have 4" or more to fit over other clothes. Men seem to like them a little looser, usually they're worn over another shirt anyway so 3 -4" positive ease is good. For kids you'd want at least 2-3", and a little more is okay because they'll grow.
sue- knitting heretic