So I finished my second interview and the interviewer said he will make a decision in 24 hours and if I don't get a call from him to call him and see

It's confusing for sure. Definitely call if you don't hear from him. Good luck!

It's a test of your actual interest in the position. And your ability to communicate. He's likely not going to call, he's waiting to see who calls back and shows real desire

Always follow up with a Thank you letter or card, mailed. Call them in 24 hours