Wenn die Katze aus dem Haus ist, tanzen die Mäuse auf dem Tisch.
I have plenty of experience of this proverb, in its literal sense.
Let's look at it word by word.
Wenn is simply when. A soundalike and lookalike word.
Die Katze is the cat.
Here aus dem Haus means out of the house.
The ist at the end of the first clause means is. So the first clause put together means: When the cat is out of the house.
The second clause begins with a verb: tanzen. This means dance.
Next we have another animal - die Mäuse. The mice.
The second clause means: the mice dance on the table.
The English equivalent of this old German proverb is When the cat's away the mice will play.
I live in an old house with plenty of tiny little gaps in the walls. Mice find it very easy to squeeze in and out of these gaps. A few years back when my dear old ginger cat Ollie died we were quite slow to find a replacement for him. Gradually we noticed increasing numbers of mice scurrying around the house. Gnawing their way through food containers and nibbling little holes into paper packages. As the winter got colder the mice got more numerous, and seemed quite well fed. We even hung Christmas chocolates up from the ceiling so the mice couldn't get at them. We now have a lovely sleek black cat who does an excellent job of keeping the mice at bay. Her name? Mouser!