This phrase means that someone has an easy life, doesn't have to work hard, doesn't have to suffer, and so on.
Older people often say this about younger people who have an easier life than they remember having:
You kids have it easy! Back when I was in high school, the teachers all used to carry big paddles around and if we got out of line, they'd whack us with them.
People don't think of "having it easy" as a very good thing. When you say this about someone, it makes them sound weak and undeserving of all the good things that have happened to them.