Each Defender Relayer is an Ethereum account assigned exclusively to your team. Every time you create a new Relayer, Defender will create a new private key in a secure vault. Whenever you request Defender to send a transaction through that Relayer, the corresponding private key will be used for signing.
You can think of each Relayer as a queue for sending transactions, where all transactions sent through the same Relayer will be sent in order and from the same Ethereum account, controlled exclusively by your team. Skip ahead to the Under the hood section if you want more info on how Relayers work behind the scenes!