You can find the answer to this question in the first chapter of the book. Enfield is tellng Utterson, who is a distant relative of his, about this strange thing that happened that has to do with this house.
What he tells Utterson about is our first introduction to Mr. Hyde. Enfield tells about this time when he saw Mr. Hyde badly hurt this little girl. He says he saw this man just run right over a little girl and trample her underfoot. When he was stopped he seemed not to think he had done anything wrong.
Enfield talks about how evil the man looked. This is our first glimpse of Hyde in this book.