What does it mean to tell a girl that you like her, that she is laughing at me to death?

I think you confessed to that girl, and then she died laughing. Maybe the girl thinks you are teasing her and joking with him. So the girl didn't take what you said seriously.

In other words, in private, your usual relationship may be too familiar, so there may be a lot of teasing and jokes. So if you talk to him seriously today, maybe he thinks you are teasing him.

I don't really care what he says. The main thing is that he didn't refuse you, so you must behave well in the future. Be sure to let the girl feel your sincerity to her, so that she will trust you more and open her heart to you.