It's a French white sauce with roux (butter and flour), milk, and (sometimes) nutmeg. The flour is cooked with the butter (also known as a roux), and then you whisk in some milk. As the sauce cooks, it thickens, and you can use it as is or add other ingredients. It's considered a mother sauce in French cooking because it's so versatile. For the recipe I'm sharing, I use some cream with the milk to amp up the richness, and then the bechamel is flavored with onion and garlic and into a creamy cheese sauce by adding shredded cheese. The key to a smooth sauce is to cook it slowly. It takes about 10 to 15 minutes for the sauce to thicken.