More specifically, it is described as achieving "the best conditions possible for a human being": happiness, virtue, morality, and a meaningful life.
This article describes a eudaimonic approach to happiness as living "authentically and for the greater good" and one that pursues "meaning and potential through kindness, justice, honesty and courage".
(Wow it's hard for me to suppress the desire to add an Oxford comma in that quote....)
Here is the Wikipedia entry, which roughly defines eudaimonia as "human flourishing"; from there you can travel down the philosophical rabbit hole, if you wish.
Etymology: Greek, eu, good + daimon, spirit