I think it starts to make sense if you break it down by word.
Permissionless - anyone can access it (e.g. a database).
Distributed - the details within the database/blockchain aren't isolated to specific places, like being held on a single mainframe, but distributed across the network of users.
Consensus - changes or operations within the database are effectively agreed by consensus. So, for example, if someone hacks a single computer and tries to change the information in the blockchain, the rest of the network will reject it and preserve the genuine database because it wouldn't tally with their own copy. It would need consensus among the network for changes be made.
Protocol - a standardised method for achieving something.
So a permissionless distributed consensus protocol would be a standardised method for the various computers / nodes in a distributed network - where noone needs permission to be able to read/write changes to the data - to reach agreement about that set of data (e.g. whether to change it or not, what to change it to etc).
Or something like that.
Bitcoin is a permissioned blockchain, so would have permissioned distributed consensus protocols, although whether or how they might be different to permissionless ones I have zero idea.