This has two meanings:
(a) The mill can be turned only by the water that is passing through it now. The water that turned it yesterday cannot be used to turn it today. The flow of time can be compared to the flow of water through the mill. We cannot use time that is past. We can use only the present time, so we must make use of our opportunities before it is too late.
(b) It is no use sighing for the past. We cannot call back happier days, but must live in the present. Nor should we allow ourselves to suffer from vain regrets. It is no use crying over spilt milk. What’s done cannot be undone.