At a very high level, this is one among many Rube Goldberg-esque interventions in financial markets that the Fed uses to try and whip markets into behaving how it wants.

In more concrete terms, banks (i.e. members of the Federal Reserve System) keep USD reserves on deposit at the Fed. The only thing they can do with these reserves is loan them overnight to other member banks at a market-determined interest rate--the Federal Funds Rate--which is targeted to a certain range by the Fed's policymaking committee. The Fed also pays interest on these reserves, at two rates: one rate for required reserves, and another for excess reserves. These serve to put a floor under the FFR, since there is no reason to lend reserves at a rate below what you can get by just sitting on them.

Of note is the fact that only Fed member banks have access to this, so other financial institutions must go through the banks when they have excess cash to park somewhere. In essence, the bank can accept overnight cash from non-banks and split the IOER with them. This transaction is consummated through a repurchase agreement (repo) in which the bank sells a "safe" asset to the counterparty with an agreement to buy it back soon thereafter (often overnight, but potentially up to a year later) for a slightly elevated price. The price difference is effectively the counterparty's cut of the IOER accrued during the time that the bank was sitting on the cash. Repo transactions are used for all sorts of short-term funding needs among non-banks, so the overnight rate on high-quality repo is roughly equivalent to the FFR.

It is for this reason that the Fed started its reverse repo operation, whereby it offers basically the same deal that I described above to certain qualified non-bank counterparties, in order to set a floor on overnight repo rates. (You can ignore the "reverse" in the name; it just means that the Fed is the one lending securities in the transaction.) The Fed is extremely wary of negative interest rates and the effect they might have on market behavior, so reverse repo appears to be the preferred method for preventing this.

So what does it mean when usage of this facility skyrockets? Well, it means that banks are not willing to engage in overnight repo at the rate that the Fed is offering, which in turn means that there is suddenly a large imbalance between repo supply (high-quality lendable securities held by banks) and demand (idle cash held by non-banks). As to what that fact means for the near future, opinions may differ sharply.

Banks essentially make money by arbitraging time preferences--they borrow short term (e.g. demand deposits which can be withdrawn at any time) at very low interest rates, and lend long term for much higher rates to risky ventures. They realize a profit by earning a sufficient spread between these rates to offset losses due to counterparty risk (i.e. default) on their lending. One consequence of this model is that a bank may abruptly become insolvent due to short term market conditions, if it cannot roll over its sources of funding. Since financial assets can typically be liquidated quickly (as opposed to, say, a bunch of idle factories owned by a defunct manufacturer) this can lead to systemic instability when an insolvent bank is forced to sell everything and drags down the prices for assets held on other banks' balance sheets.

After the GFC, regulators decided to come up with a more nuanced set of rules about how "healthy" a large bank's balance sheet must be, in order to spot trouble before it exacerbates a liquidity crisis and produces a solvency crisis. A business's leverage ratio is basically capital (equity) divided by assets (or its inverse, depending on your framing). For banks, however, just looking at leverage is not that helpful since the assets being held have very different levels of risk. The new metric is the Supplemental Leverage Ratio (SLR), which includes off-balance sheet exposure. Notably, the bank's reserves at the Fed as well as holdings of US Treasuries are normally included in the denominator (risk assets), but at the start of the pandemic an exemption was put in place so these could be excluded, thereby boosting the leverage ratio and allowing banks to engage in more lending than would otherwise be allowed by the normal SLR calculation.

However, the SLR exemption has now been allowed to expire, and thus banks must tighten up their balance sheets to avoid the severe restrictions of a low SLR. We are now squeezed between the Scylla and Charybdis of financial regulation and monetary stimulus, as the Fed engages in QE to encourage lending towards riskier economic activity while simultaneously imposing leverage constraints to prevent large banks from posing systemic risks. The Fed's reverse repo has become the pressure release valve, as banks are completely hamstrung by their inability to offer negative rates so everyone is now going straight to the Fed for their overnight deposits.

Ironically, this is essentially a (short-term) negation of the Fed's QE activity, as the Fed is simultaneously purchasing assets as well as lending them out on an ongoing basis. In other words, the market is sending a pretty clear signal that QE is really unnecessary at this point in time.