Classroom management is a critical focus of effective instruction because there is no way that effective teaching can happen in a poorly managed classroom. Even if a teacher has a tremendous grasp of the material and has wonderful lesson plans, there is no way he or she can get students to learn much if the classroom is not well managed.
Classroom management refers to the process of keeping order in the classroom. It refers to the process by which a teacher ensures that there is an environment in which students can concentrate on learning. If such an environment does not exist, no real learning can happen.
For example, imagine a classroom in which the teacher cannot control the amount of talking that is going on between students. There will be background noise that prevents students from attending to the teacher. There will be many students who are not even trying to listen to the teacher. This will mean that hardly anyone in the class will be concentrating effectively on the lesson that is being taught.
A classroom needs to be orderly to allow students to pay attention to the teacher. Therefore, classroom management is a critical focus of effective instruction.
Classroom management is critical for learning for several reasons. The above answer covers much of what a teacher must know about classroom management. I agree that it is critical because without order, students cannot learn. I would add that classroom management is necessary for students to feel safe, both physically and educationally. The year I broke my ankle and was out for six weeks; my students did not do well with a substitute unfamiliar with the vagaries of eighth graders. One of my female students was under verbal and finally physical harassment until she went to the dean in the office. Two students were expelled for their actions. She definitely did not feel safe. If the classroom were under better control, the problem would have been noticed and dealt with long before it got as far as it did. Another issue is the emotional safety of being able to express an opinion and expect that any challenge to the opinion will be done without attacking the person. Giving a wrong answer is another way students need to feel safe--that no one laughs or makes fun of the student because we are all in this learning process together. All of us make mistakes and if students can grasp the idea that at some point each of them will make a mistake and other students will treat them respectfully, then the environment within the classroom is safe to either give answers or ask questions.
Another reason why classroom management is so critical is the waste of time which can result from poor management. With all of the state and federal tests to be taken, and the regular curriculum to teach, time is so important. Students who will not conform at all so that others can learn must be dealt with as soon as possible. Perhaps there are medical issues, a class before mine which allows them to interrupt this much, or maybe some students have never understood that the word NO applies to them as well. Whatever the reason, classroom management needs to be a primary concern for a teacher, especially a new teacher. Ask for help if you need it from a mentor or a teacher you respect.