I have been a regular reader for over a year and have read all of the sidebar material. This place is a goldmine of truth.

BUT one thing I keep reading over and over and despite reading all the relevant article about it by CH, Mr Tomassi and others and many many posts here... I simply do not grasp. Every discussion about it is so vague it is essentially worthless. I am sure many others feel the same way. It would be very appreciated if someone can concretely and concisely explain this concept:

What is frame? How is it controlled/maintained?

I just want the basic definition, please no irrelevant philosophical discussion.