Fediverse is like the United Nations and instances are the countires. Countries with good relations communicate (federate) with each other but the ones that don't respect our values we might cut communications with (defederate). These defederated countries still continue to exist and there are people within those country that talk with eachother aswell as other countries they do get along with but they're essentially secluded from the rest of the world. However despite the defederation they still are in principle compatible with the rest of the world so we can open back the communication again if we so decide.

Other social media platforms like Instagram and Twitter are like the animal population; they too communicate within themselves but we don't have a common language and thus will always remain incompatible and can't ever federate with them and they (Twitter and Instagram) can't with each other either.