What is a Finance Charge? Are auto insurance companies supposed to charge it?

I have AAA insurance for my car. I had them for last year and they were fine, I paid the same amount every month. Now they sent me my renewal bill and they are telling me that if I don't pay my premium in full in one shot, I will be charge a $ 50 something dollar finance charge and an APR of 18%. What does that mean? Do all auto insurance companies do that? Thanks