What determine a form / records is on stage dirty

Hi guys,

I'm not sure I said it correctly, as I'm new to this CRM (Model-driven App). But there is some information I read about form dirty checking, it is something about the data has changed, so my question is actually what is that, in a simple explanation ?

And if I have field that at first is null, and I put a Business Rule in the background to put some value, will this process make my form is dirty as well ? or is it only when it has data and now is changed ?

Hope it's clear.

Thanks