I'm learning about interest (simple and compound interest). I don't understand it. What is interest exactly?

I know the formula for simple interest is
I = prt
And then you have to add the interest to the original amount.

I learned that the formula for compound interest is
M = p (1 + i)ⁿ
But when I searched it up, it said it was
A = p (1 + r/n)ⁿᵗ

Can someone please explain this all to me ASAP? I need to understand this all before tomorrow.