少しも～ない is a fixed expression used to negate something completely. It is stronger than just saying 全然～ない, and often carries a nuance of surprise or disappointment. For example, if someone asks if you are tired and you are not, you can say 少しも疲れていません, meaning you are not tired at all, not even a little. It is often used with statements of degree or feeling, such as 寒い, 疲れる, 心配, or 面白い. In conversation, it can be quite emphatic, so it fits when you want to stress that something is completely absent. While 少しも is not as formal as some N1 patterns, it is still more common in written Japanese and in careful speech than in casual chat. In casual talk, people might say 全然～ない instead. Note that 少しも must be followed by a negative form; you cannot use it with positive verbs.