From the link

This series is constructed as the aggregated daily amount value of the RP transactions reported by the New York Fed as part of the Temporary Open Market Operations.

Temporary open market operations involve short-term repurchase and reverse repurchase agreements that are designed to temporarily add or drain reserves available to the banking system and influence day-to-day trading in the federal funds market.

A repurchase agreement (known as repo or RP) is a transaction in which the New York Fed under the authorization and direction of the Federal Open Maker Committee buys a security from an eligible counterparty under an agreement to resell that security in the future. For these transactions, eligible securities are U.S. Treasury instruments, federal agency debt and the mortgage-backed securities issued or fully guaranteed by federal agencies.

As per my understanding, banks ran out of cash to loan so the feds gave them some with the theoretical agreement that the banks will pay them back later.

Stonks