A Fedimint federation is a group that jointly controls a Bitcoin multisig wallet and enforces certain rules on the funds inside. As long as a majority of the group is honest, the federation functions correctly. This additional trust and security tradeoff allows adding versatility to Bitcoin that would not be possible otherwise.
Rebranding MiniMint to Fedimint: The most visible change has probably been the renaming of the open-source project from MiniMint to Fedimint. Initially, Fedimint referred to the abstract concept, while MiniMint was the concrete implementation built by independent developers in collaboration with the Blockstream Research team. The dual names have been a source of confusion, thus a decision to unify the branding.
By now, Fedimint has all core user-facing features in place, and the goal is to release a signet demo soon, but there is still so much left to do:
Improving the resilience and performance of the consensus algorithm
Increasing test coverage
Building new and exciting experimental federation modules