How to understand informal and formal - General Linguistics class?

I have homework due Wednesday and I just want to finish it all up before the weekend if I can. I was in General Linguistics class and the teacher told us to re-write Eminems´ – “Not Afraid” into informal, I think sorry I have bad cramps and I don’t feel like standing up and look it up in my diary. I don’t quite understand the informal and formal part, I am also using online dictionary right now but it’s all still complicated and stuff. Can you explain it to me in a much easier way?
I know like informal is related to how you talk to older people and stuff like how you would approach your grandparents is much different that how you’d approach your friends.

It’s a little complicated for me so I want to spend the weekend teaching myself it…