It means to take advantage of a situation and profit from it - usually an unrelated event.

To "cash in on" something or a situation means to get some kind of gain or advantage from it, usually a financial one, to profit. You can cash in on your beauty, on your husband's fame, on a new trend, on your experience etc. If you just say "cash in" that means to exchange something you own for cash, essentially to sell it: cash in your casino chips, cash in your stocks etc.

maybe take advantage, thanks