Base money (both cash and central bank reserves) is often emitted through short-term repo operations. If a bank wants some base money (e.g. to put some banknotes in ATMs), it sells central bank some (usually government) bonds with promise to repurchase it after a fixed time limit (e.g. 14 days, done in periodic rounds). That is essentially a lease agreement with different legal detais.
So it is essentially an IOU for bonds, just not redeemable in retail (by individuals) but in bulk (by commercial banks). If commercial bank wants 'redeem' some cash (for bonds), then it just acquire less cash in the next round of repo operations.
It's an IOU for 100 dollars. You can exchange it for other cash IOUs from the central bank. A bank can also exchange it for electronic central bank reserve IOUs.
So it is essentially an IOU for bonds, just not redeemable in retail (by individuals) but in bulk (by commercial banks). If commercial bank wants 'redeem' some cash (for bonds), then it just acquire less cash in the next round of repo operations.