The white sauce is used all over the world but is widely known as Béchamel, a standard sauce in French cooking.
Simply, it is fat + starch + liquid dairy or substitute. The usual proportions are one part butter, one part flour and enough milk or dairy substitute added gradually until desired thickening occurs. In case you are interested, this is called a Roux.
Melt approx 1 tbsp butter on low
Add same amount of flour
Mix to a paste
Slowly add milk, whisking until sauce thickens
Add salt, pepper, garlic powder, mustard, cheese, herbs…anything you want.