Be prepared for disasters that have not yet occurred and plan everything.

Managing strategies means that a person is very capable and can always control the overall situation no matter what problems he encounters. Preparing for a rainy day means making plans before things happen.

The whole sentence is to prepare for a rainy day, strategize, and win a victory thousands of miles away. This famous saying comes from the Historical Records of the Great Ancestor. It usually refers to the ancient generals who could predict the situation of the battle thousands of miles away while sitting in the military tent.