As the saying goes, it means that you have done your best, and whether you can achieve your goal depends on your luck. The phrase comes from the 103rd time in the Romance of the Three Kingdoms written by Luo Guanzhong in the Ming Dynasty: "Kong Ming sighed: 'Conspiracy is in man, success is in heaven, and it is not strong!'" ’”
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In English, it can be expressed in Man proposes, God disposes, or The planning lies with man, the outcome with Heaven.
Example:
We have done our best for this project, and now it is up to us to see if it will succeed. The plot is in man, and the success is in heaven.
We've given our very best to the project, and, at this point, it's up to fate whether things work out for us. Man proposes, God disposes.
Source: China Daily