Used to express that something can't be fixed and should be replaced. My old boots have had it; the soles are falling off.
Pro Tip 2/3
Not About People
It usually refers to objects or situations, not people, unless used humorously. After the long hike, jokingly, I said I've had it and needed a new pair of legs.
Pro Tip 3/3
Emphasizing Finality
Highlights that there's no hope of returning to a good state. This jacket has had it; it's ripped everywhere.