You should always wear a tight seatbelt because it could save your life.

tight 是形容词，意思是"紧的、牢固的"，描述安全带需要系紧的状态才能起到保护作用。 seatbelt 是复合名词，由 seat（座位）和 belt（带子）组成，指固定在汽车座位上用于保护乘客的安全带。