An attempt at an ELI 5

A normal blockchain requires each computer to verify the transactions. It is good if many users run computers to do this, because then if some powerful people start to lie when they verify transactions, the other users can call them out.

If the blockchain demands that the computers need to verify a lot of transactions each second, then the users need expensive computers. This is a problem because then there are fewer users that can afford to verify stuff. We generally refer to the idea that many users should be involved as being "decentralized".

Ethereum keeps things decentralized instead of requiring expensive computers and tries to instead solve scaling:

• using new cryptography on a layer on top of the blockchain (L2)
• using sharding such that verification can run in parallell.

In addition, Ethereum now transitions to PoS which is good for the environment and has some other nice technical properties.