This phrase is derived from The Art of War authored by Sun Tzu (《孙子兵法》Sūnzǐbīnɡfǎ), “to be near the goal while the enemy is still far from it, to wait at ease while the enemy is exhausted, to be well-fed while the enemy is famished---this is the art of husbanding one's strength.(以近待远,以佚待劳,以饱待饥,此治力者也。)” In 684 BC, the Qi army attacked the state of Lu. Lu Zhuang Gong (鲁庄公 Lǔzhuānɡɡōnɡ) then decided to embark on an expedition with his warriors. Cao Gui (曹刿 Cáo Guì), a warrior went with him. The Qi-Lu parties set the battlefield in Chang Shao (长勺 Chánɡsháo). Qi took the lead to beat drum. Soldiers rushed over like a flood. But Cao Gui persuaded Lu Zhuang Gong just to keep up with a good defense. Qi began to launch the second round of attacks. They beat the drums again. But Lu stayed on the defensive and didn't attempt to strike. Because of this, Qi thought Lu was timid and then relaxed. When Lu heard Qi beating the drums for the third time, Cao Gui commanded their troops to attack. The Qi army was exhausted after two attacks. Meanwhile, the Lu soldiers had been in resting for a long time and were finally ready to attack (蓄势待发 xùshì-dàifā). As a result, the Lu soldiers beat the Qi army and won the battle.
The idea is to have your troops well-prepared for battle, in the same time that the enemy is rushing to fight against you. This will give your troops a huge advantage in the upcoming battle, of which you will get to selec the time and place.