Ethersjs Infura contract connection

I am trying to integrate ethersjs with Infura from my react app. I see in Infura documentation at https://docs.infura.io/infura/tutorials/ethereum/send-a-transaction/send-a-transaction-2 they have mentioned to create .env file and add the following details

ETHEREUM_NETWORK = "sepolia"
INFURA_API_KEY = "<API-KEY>"
SIGNER_PRIVATE_KEY = "<PRIVATE-KEY>"

Who is signer here? Whose private key should be used here? In earlier version of ethersjs we would just use provider.getSigner() to get the signer.