A fork would be a duplicated custom copy of the software that can have its own changes or improvements added and is usually maintained by a different person/group than the original. When a new update of the original is released, the maintainer of the fork can bring those changes into their custom fork.
Instances are (mostly identical) copies of the same fork. They’ll have custom names and different logos, but the software that’s running them is all version 0.18 of the same fork. They may install updates at different speeds ie v0.19 is released, some instances will update immediately, some may take a week, but eventually they’ll all be updated to v0.19.