what is idioms and give example of idioms and their meaning

Idioms are basically phrases that mean a certain thing. You can search online for more idioms.
Some examples I know are "back to square one" (Back to the beginning/from the start), and "beating around the bush" (Not talking about the main topic[or something like that])
1.) It was raining cats and dogs yesterday.
2.) Cecilia was feeling under the weather.