The idea
A ruler through a cloud of dots
Plot every day of the bakery: cookies sold across, coffee sold up. You get a cloud. Now lay a ruler through the middle of it so the dots sit as close to the ruler as possible.
That ruler is a regression line. It answers one question: if I know the cookies, what is my best guess at the coffee?
Coffee = intercept + slope × Cookies
Nothing about causes. The line says the two move together. It does not say cookies make people buy coffee.