Delayed cord clamping is, simply, not cutting the umbilical cord after birth till it has done its job.
Your placenta and baby are one functioning unit; the placenta acts in a similar way to a kidney dialysis machine by filtering the baby’s blood and returning it to the baby’s body. At birth, the placenta is still full of your baby’s blood.
Cutting the cord early means your baby may lose up to a third of its own blood supply which is still in the placenta. Cutting the cord immediately means baby’s body is immediately at a disadvantage, fighting to do all the things it needs to do after birth without full blood supply.
A person on a kidney dialysis machine would not be disconnected from the machine till all their blood was returned to their body; why disconnect a baby early?