level: Level = Level.ERROR,
    changesMayBreak: Array<Impact> = arrayOf(Impact.COMPILATION, Impact.LINKAGE, Impact.RUNTIME))

Signals that the annotated annotation class is a marker of an experimental API. Any declaration annotated with that marker is thus considered an experimental declaration and its call sites must accept the experimental aspect of it either by using UseExperimental, or by being annotated with that marker themselves, effectively causing further propagation of that experimental aspect.