To make it is to accomplish something, or to become successful. Your college advisor might tell you that to make it in the physics department, you've got to be good at both science and math.
An actor might try to make it in Hollywood, while a young dog lover may hope to make it in the world of dog shows. You can also use make it to mean "pass," asking your friend, "Did you make it?" and having her answer, "No, I have to take the driving test again next month." And when you just barely make it, you survive: "He's trying to make it on a librarian's salary."