The interest your CPF Ordinary Account would have earned if you had not withdrawn it to pay for property. When you sell the property, you must refund both the CPF principal used and this accrued interest back into your CPF account, not into your pocket.
Example: If you used S$150,000 of CPF for your flat over many years, you could owe tens of thousands more in accrued interest on top when you sell, reducing the cash proceeds you walk away with.
It is returned to your own CPF account, not lost. The refunded amount then continues to earn CPF interest and counts toward your retirement savings, so it is not money gone, but it does reduce the cash you receive from the sale.
Because the refund (principal plus accrued interest) is deducted from your sale proceeds before you see any cash, a property can look profitable on paper but leave little or negative cash in hand once the CPF refund is made. This is sometimes called a 'negative sale'.