Au Lait means "with milk". Cafe au lait is a simple and quick French-inspired coffee drink made with milk and coffee. It can be served hot or cold, depending on the weather and your taste, and you may add sugar to it if you like it sweet or salty if you enjoy your coffee with a little salt.
Though cafe au lait usually refers to the drink's preparation (with milk added to strong black coffee), there are cafes which serve their own version of this beverage by adding frothed milk instead, but we will stick to the original for now.
The origin of cafe au lait dates back to the middle ages in Europe. It was a beverage made for adults and mainly consumed at breakfast time. That's why cafes started serving this coffee drink on their menus with bread or croissants. In France, cafe au lait is usually accompanied by brioche or croissant with butter and jam, which you need to dunk into your cup before drinking.
Nowadays most people drink cafe au lait just because they enjoy it, but in the past this coffee drink was believed to be beneficial for curing colds. The milk added into coffee was said to improve the body's resistance against diseases and infections, while stimulating brain activity. This is probably why French doctors advised their patients to drink cafe au lait after receiving treatment!