You’ll hear this on the news sometimes. When a country is in turmoil, and the legal or constitutional government is out of action, we talk about whoever is now in charge as the de facto leader, or the de facto government. They are in charge not because of a legal process, but simply from the fact that they have the power.
De means from, and now you know it takes the ablative.
De also means about, so a lot of Latin prose works start with de: De Providentia, About Providence. De Senectute, On Old Age. De Ira, On Anger. De Otio, On Idleness.
We will get to some of them in time.