What does this mean? It means that we should do the appropriate job at the appropriate time.
When you have to study, you should not do something else and waste your study time. Similarly when it is the time for you to play in the evenings, do play at least for one hour as your body also requires exercise. Just keep on studying you will get bored.
All of us should do the proper work at the proper time. When it is time to go for work, we should attend our work sincerely and not skip or miss our work. Similarly if it is the time to play or relax, you should go to play and relax.
So this saying has got good meaning, work while you work and play while you play. In simple perform your work at the appropriate time.