After being accepted, you have the right to complete your desired degree. If you have been accepted through the main admissions process (yhteishaku), this means both a lower (bachelor’s) and higher (master’s) degree; if through separate admissions for international master´s degree programmes, a master’s degree only.
To keep your right to study, you have to register every year as either attending or non-attending. Only attending students are entitled to financial aid and to having credits entered on their record. Your university user ID will work only if you are registered.