(one) won't bite (someone)

Someone has no reason to be afraid of one; someone should not be shy about approaching or talking to one. Don't worry, child, I won't bite. I just wanted to give you a message to pass along to your father. This is your Uncle Jack, son. Well, go on and shake his hand, he won't bite you. Well, come on in, I won't bite you! Actually, I have very good news for you—you got the promotion!

won't bite

Is not threatening or does not need to be a subject of caution; is friendly. The phrase is typically used as an assurance to a child who is wary of something or someone. You can ask Mr. Caruthers. Go on, he won't bite. Hi there, what's your name? Don't worry, I won't bite!