Need help understanding...

I need help from the pros!

I guess I just don't understand the TDEE/BMR/RMR/etc things. I need someone to explain it so that I can understand what's best for me to do. I've tried the google and I'm still having a hard time grasping. MFP has me set up for 1340 calories a day and that just seems really low. Maybe I'm just used to eating all the food but I don't feel like I'm eating enough. Maybe MFP calculations are right and I need to get used to eating less or maybe the TDEE/whatever way would be better for me but I have no idea. I need someone to explain in lay terms. Please and thank you.

ETA: I know consistent logging is important. I know I am not very consistent.

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