It is like "pick and choose". Just like on a menu...I will have a burger, some fries, a soup...a little of whatever you want rather than a "meal" that will automatically come with salad, soup, entrée, etc. already "formulated" for you.
... you don't exactly have to know French to understand à la carte. It's used in every greasy spoon in America.
Why aren't there menus in America? I love how in France they give you a limited, but well-rounded, selection of meals. I hate being overwhelmed by having to pick and choose everything myself à la carte.