It's the Fed (singular), short for the Federal Reserve, the US central bank. The Fed has unlimited ability to lend money and buy and sell securities. It electronically creates the dollars to do this. Normally it uses this power to control the short-term interest rate (which determines how cheaply businesses and consumers can borrow). But especially in a crisis when it has already taken the interest rate to zero, the Fed can take additional steps to stimulate other markets.
The Fed "saves the economy" by ensuring there is plenty of money and demand for financial assets (liquidity). This "monetary stimulus" can mitigate crashes, but it doesn't always do much to spur real economic activity. It works best if combined with "fiscal stimulus", or money paid outright by the US Treasury to businesses and consumers via tax cuts or spending, but fiscal stimulus is harder to enact because it requires cooperation of Congress and the President, whereas the Fed is independent and can act quickly.
The Feds (plural) can refer to the US federal government broadly (though most often to federal law enforcement). Occasionally someone might refer to overall economic policy made by the "Feds" (which could include the Fed along with Congress, the Treasury, the White House, etc.). But the most common usage in finance is talking about the singular Fed.