In most English-speaking countries outside North America, an entrée is a smaller course that precedes the main course. In North America, this course is typically called the starter, with entrée used as a synonym for the main course.
An entrée is more substantial than hors d'œuvres and better thought of as a half-sized version of a main course. Restaurant menus will sometimes offer the same dish in different-sized servings as both entrée and main course.
The word entrée comes from French where it means "entry". Used in English, it literally describes a meal's first dish.