In English, there have been lots of idioms in the past and there will be lots of idioms in the future. A specific idiom can become old. There are many idioms that my parents and grand-parents used that are not used by speakers today. And there are many new idioms that are used today that were recently created. Sometimes, it is difficult to stay up to date, dropping old ones and picking up new ones. Was the YouTube teacher talking about specific idioms?

Idioms are expressions that have been in use for a long time and take on a different meaning than what the individual words actually mean. It’s raining cats and dogs = there is a torrential rainstorm Are you under the weather = are you feeling ill