This is something that has always seemed pretty simple to me, but I feel like a lot of people don't get what it means for an idea to be "canon-friendly". Let's break this down.
Canon / noun / a general law, rule, principle, or criterion by which something is judged.
In the case of 40k, this means the current state of the lore as established by the latest releases from Games Workshop and Black Library.
Friendly / adjective / showing kindly interest and goodwill; not hostile.
So, what is "canon-friendliness" and what are its limits? Canon-friendliness is essentially compliance with the canon, meaning that everything should feel like it fits in with established material. Nothing should feel forced or out-of-place. For instance, you could say that a planet was inhabited solely by ponies until they were all eaten by tyranids. Does this directly contradict anything written in the canon? Probably not. Is it canon-friendly? Nope. Ponies have no place in 40k, nor should they. Adding them in ruins the feel of the setting, therefore making it NCF.