This is quite a long phrase which is probably self- explanatory. It simply means that, without some authority or control, people will take advantage of a situation.
The boss is away this week so everybody’s taking it easy but as they say, while the cat’s away, the mice will play!
Here’s an example from a popular Scottish comedy series called Still Game. You may find the accents a bit of a challenge but it’s good practice.
A landlord is the name for the person in charge of a pub. If he goes away, the brewery (beer maker) bring in a relief manager. In this scene, the locals of the pub trick the relief manager and take advantage of the manager’s absence.