Biens mal acquis is a term that refers to assets or properties that have been obtained in an illegal or unethical manner.
To explain this like you're 5 years old, imagine you have a friend who took a toy from another friend without asking. This would be wrong and not fair, and the toy they took would be a "bien mal acquis."
Now imagine that instead of a toy, someone took a lot of money without asking or through illegal means. This money would be a "bien mal acquis" and it would be wrong because they didn't earn it fairly.
In real life, sometimes people who have power or are in charge of things, like politicians, take money or property in a way that is not honest or legal. This is called corruption, and the things they take are "biens mal acquis".
When people talk about "biens mal acquis" in a political sense, they are usually talking about trying to recover this money or property so it can be used in a fair and legal way. It's like telling your friend they need to give back the toy they took so everyone can play with it fairly.