not knowing the immensity of heaven and earth; being ignorant of the world's complexity or scale

他初出茅庐，不知天高地厚，总以为成功是件轻而易举的事。

having an exaggerated opinion of one's own abilities; being brash or arrogant

那个拳击新手不知天高地厚，竟然敢向前任冠军公开叫板。