What is meaning of magnitude??
Think about when you woke up today. In all likelihood, you were probably woken up by the sound of an alarm clock. The loud sound of the alarm was the cause. Without the alarm, you probably would have overslept. In this scenario, the alarm had the effect of you waking up at a certain time. This is what we mean by cause and effect
In physics, loudness of alarm is the quantity and how loud it will ring is defined by its magnitude. So, in simple terms, magnitude is the extent to which a cause will effect any system. For example, how hot water should be to vaporise (100℃ - 100 is magnitude ). Or how hard should I beat an iron rod with hammer in order to break it ( some X newton ).