Since there's little context to this, the banks in question used the excess in deposits during the pandemic to buy up bonds that had extremely low interest rates and wouldn't mature for years to come.
Then when interest rates started spiking, they didn't have liquidity to invest and aren't making enough off those low-interest bonds to pay interest on the deposits they're holding.
The $517B number being thrown about here is the sum of the difference between what they're earning on their investments in bonds and what they're paying in interest to their depositors over the past 3 years.
