Still a bit confused about hextech crafting.

I know there is a million page Q&A but I don't feel like scrolling trough them all for a tiny piece of info. I don't understand how the mastery system affects the drop rate. Sure, it's clear that getting an S grade yada yada but people kept mentioning 5 star mastery. Are you only eligible for a drop if you or a pre get an S grade on a champ with 5 star mastery? Do you loose the chance for a drop with a champion when you reach 5 star mastery? I just don't get it and people mention it a lot, ofc not in a context that would help me understand. Maybe there was some link with info I missed from the go?

It won't affect my choice of champions (buying them all and getting 5 stars in alphabetical order to keep myself away from ranked) but not knowing something everyone else does annoys me.