What are trusted repositories ?

Hi there, In some review, some has mentioned the notion of "trused repositories". Can we have clarification on this notion and what we expect for this ? Can we trust proprietary repo ? What can be audited (even when closed source software). What level of trust can we expect or not ? At least previously we had a page listing some repositories that would be compatible: https://rpmfusion.org/FedoraThirdPartyRepos (linked from the fp.o page). Thx -- - Nicolas (kwizart)