I believe@shadders talked about them at the conference. I think they’re like hosted message queues as a service for bitcoin related messages in case you happen to be offline etc. Like smtp servers.
They are somewhat explained in the whitepaper, by CSW, and thru the demonstrations by Shadders in the past year.
This is the way to scale transactions by passing them between parties. One side will have to send the final transaction on for settlement on chain. You can have somewhat of an assurance that the transactions will be valid by comparing the merkle proof to a copy of the block headers. This is really only valid during times where re-orgs aren't occurring... it gets complicated. A lot explained here by Attila too: https://bitcoinfiles.org/t/77a2928ada60ee24f012a5309f221c52a1b9e15a900a9bb63993340e596b0c15