"Eu" is a prefix used in biology. The prefix "Eu", coming from the Greek word for "good", is used in many different fields, but it has a special meaning in biology.
"Eu" is translated many ways, although its basic translation is given literally, as "good". However, that may be a bit misleading in biology, and a more accurate translation might be "correct", "legitimate" or (my preference) "complete". A eukaryote is a "complete kernel", a "eutherian" is a "complete mammal" and a "eumetazoa" is a "complete animal".
While the difference might seem slight, I think it is important to note this because the word "good" has connotations of morality that have little meaning in biology. In other words, no one should feel that because a kangaroo is not a eutherian that it is somehow morally compromised.