What happens if you tell a mathematician that you are worried because you have 1 in 3 (33.33%) chance of being raped?
They will tell you that if a rapist attempts to rape you three times in a row the chance of NOT being raped equals
(2/3) * (2/3) * (2/3) = 29.63%.
So your probability of being raped at least once is
100 - 29.63 = 70.37%.
Listen up, buttercup. So, this joke is saying that some mathematician hears that you're worried about getting attacked. Instead of, like, calling the cops or giving you a hug, they start doing math to figure out the odds. Because that's helpful, right? Basically, they turn a serious problem into a nerdy equation. Which is not helpful at all for a bozo like you.
