就是英语的俚语，成语的意思。比如 a piece of cake就是小菜一碟的意思。对增加英文口语很有帮助。可以在网上查

An idiom is a widely used saying or expression that contains a figurative meaning that is different from the phrase's literal meaning. For example, if you say you're feeling “under the weather,” you don't literally mean that you're standing underneath the rain.