It means that they don't get double taxed on the same income. The US has the same deal. They have tax treaties with most nations such that they give you credit for the tax you paid in the other country but then "make up the difference" with their own taxation.

For example, say you make 100k, you live in a place with 10% tax and france's tax is 50%. That's less than half the french tax level so this new law would apply to you.

You pay 10k to your country of residence. France says you would have owed 50k had you lived in france. They give you credit for the 10k you already paid where you live and tell you to pay 40k to them (for roads, obviously).