What is the difference between Relayer and Builder post Merge in Ethereum?

I was listening to an episode of Bankeless where they were talking about things coming up, post merge. In that episode they mentioned block builders and relayers.

The context they mention it was something like, right now there are block builders and relayers that a validator can offshore the task of building blocks to, but that in the feature there won't be need for relayers.

That got me a bit confused. What exactly are block builder and relayers? How are they different? And what future change are they talking about when they mentioned that relayers might not be needed in the future?