A milieu is a surrounding culture. Your family, house, neighborhood, school, and people you hang out with make up your milieu. If you live in a mansion and have rich friends, you’re part of the upper crust milieu. Go you.
A milieu is both "surroundings" and everything that makes up the surroundings. Your milieu is your world, or the context you come from. Sometimes a milieu shapes a person, as when a "milieu of abuse and poverty," inspires someone to improve things for others. Milieu sounds a lot like mildew without the "d," but unless you grew up in a moldy bathtub, mildew probably has little to do with your milieu, or environment.