BMR and TDEE?? Help!!!!

So I have been following MFP's guidelines and keeping it to 1,200 cals a day. Then I read the thread, very quickly, regarding BMR and TDEE and I'm totally confused. I know what they stand for, and I calculated my numbers, so now what do I do? What should I really keep my cals at?? I'm sure I could research it but I'm about to leave work, and don't really use the computer at home. So if someone could give me a quick explanation of what I need to do with these two numbers I'd appreciate it! I just don't have time right now to read every single post and all the info. Thanks!! I'm just bummed because I thought MFP was keeping it nice and easy!