Relays:
Validators can use Relays to outsource their Block Production to entities specialized in extracting extra revenue. These Relays exists a means of minimizing required trust under the participating entities - they only have to trust the relay itself, and not each other.
Extra Revenue is generated by reordering and/or inserting transactions in an otherwise normal block. This is often referred to as MEV, or "Maximum-Extractable-Value". The validator which gets to propose a block by a relay will get a cut of this revenue in exchange, shown below as the "block reward".
A Relay can consist of a single builder, in which case the relay builder will accept transaction bundles from searchers, or many builders, where the relay operator will pick the the block of the builder with the highest block reward.