I guess I am really out of touch but I understood about two and a half words of what you just said.

It's a bit like having an in-mem db that then commits your transactions to the actual DB which is much slower.

It means that everything you need to arrive at a consensus on the validity of a transaction is on the blockchain, and thus enforced by the decentralized network of nodes that validate the blockchain.

So contrary to what the grandparent comment claims, these layer 2 (i.e. scalability) solutions make no compromises on decentralization, as they would if they had a consensus-critical component that was managed by a centralized off-chain party.

Are you referring to the likes of Polygon / MATIC?

Polygon has several products. Usually when people say "Polygon" they are referring to Polygon's popular sidechain product, but Polygon's Hermez product is a zk rollup like I was talking about.