I don't quite understand this concept yet but is this term related to this question/answer?
I believe you're talking about 2 different types of relayers. A relayer as a general term is a 3rd party that relays some information from one party to another.
In 0x, the relayers are collecting off-chain orders people send them, and
relaying them to parties that request these orders to possibly fill.
When talking about meta-transactions, (e.g. a 3rd party submitting transactions), you have your account be a multisig, and you sign a message that the multisig can essentially execute as a transaction. After you sign it, you send the message to a relayer who bundles that message into an actual transaction. They then
relay that transaction to a public node so the transaction can be mined.
The term works perfectly for 0x, because they are simply relaying information from one party to another, but it breaks down a bit for meta-transactions since instead of just relaying, they're wrapping the signed messages into transactions and submitting them to the network. However, I can't think of a better term for them right now, so relayers it is!