It means to put someone in charge of something they are likely to abuse or destroy.
Here's why:
* Foxes are known for preying on geese. They are natural predators, and the instinct to hunt geese is deeply ingrained in them.
* Geese are vulnerable to foxes. They are relatively defenseless against a fox's cunning and speed.
Therefore, putting a fox in charge of guarding geese would be a recipe for disaster. The fox would almost certainly take advantage of its position to kill and eat the geese.
The idiom is used to describe situations where someone is given responsibility for something they are likely to misuse or damage. For example:
* "Putting the company's finances in the hands of a known gambler is like setting a fox to keep geese."
* "Hiring a notorious gossip to manage the office newsletter is like setting a fox to keep geese."
The idiom highlights the absurdity of putting someone in a position where their inherent nature conflicts with their supposed duty.