this doesn't make any sense
Actually it does. You see, infinity is not a number, it's a concept. It's a concept of never-ending, bigger than any finite number.
This means that you can do almost anything to it. Some properties include:
∞ +- (Any finite number) = ∞
∞ - ∞ is undefined, ∞ + ∞ = ∞
∞ * (Any number or ∞) = ∞
∞ / (Any finite number) = ∞, ∞ / ∞ is undefined
(Any finite number) / ∞ = 0
(Any finite number)
∞ = ∞, ∞
(Any finite positive non-zero number) = ∞
1/0 ≠ ∞