套子 is a noun with both concrete and abstract senses. Concretely, it refers to any kind of covering, case, or sheath that encloses an object — like a knife sheath, a phone case, or even a condom. Abstractly, it means a cliché or a formulaic, unoriginal phrase or way of thinking — often used in 老套子 (old cliché) to criticize something as stale. A third, less common sense is padding or wadding material used in clothing or packaging.