Go to a fancy restaurant and take a look at the menu. If you decide to order the chicken, you've just selected your entree.
Entree, pronounced "ON-tray" and sometimes spelled entrée, is another word for main course, at least as it's usually used in this country. Sometimes people talk about having an entree into society, or into some club. When it's used in those contexts, it usually means you have someone who's going to smooth the way, introduce you, and allow you to enter.