A homecoming party is an annual event, soon after the beginning of the school year, at many high schools and colleges. It usually consists of some kind of party with entertainment and in many cases a parade and/or a sports event (in America, usually an American football game between the school and one of their main rivals). The events, inculding the sporting event, usually last over a few days on the weekend and are in celebration of "coming home" again. In this case "coming home" means coming back to school. So it is a celebration of the new school year.