It's how you look at population changes in biology. Goldfish in a pond, say. This year there are x goldfish. Next year there'll be y goldfish. Some get born, some get eaten by herons, whatever.
Nature manipulates the x and turns it into y .
Then y goldfish is your starting population for the following year. Just like Thomasina. Your value for y becomes your next value for x . Whatever it is, it can be written down in mathematics. It's called an algorithm.