hi guys! anyone can help me? please tell me about informal idiom ? thanks in advance

idiom form  is like a part of all form of expressions both formal or informal. but some idioms best suited in formal contents. whilst other are best for informal ones. the use of informal idiom outside its costumary level of formality is often an indicator either of the some author's degree of respect of formality. or his level of literacy. some people usein their appropriate circumstances. while the other may deliberately miss-use it to strengthen the expressions of their personality prejudice. (thats all i know)