An adverb used with negative sentences to emphasize 'not at all' or 'not even a little'. Rarely used in affirmative sentences.
彼は少しも疲れていないと言ったが、顔色は悪かった。
He said he wasn't tired at all, but he looked pale.
宿題は少しも終わっていない。
I haven't finished the homework at all.
彼女は少しも驚かなかったようだ。
She didn't seem surprised in the least.