What's "cornerstone" mean?

Native speaker’s answer
Rebecca
"Cornerstone" means an important quality or feature that something is based on. It comes from the "cornerstone" of a building, a stone placed on the corner as one of the main foundations. Here they're saying that email has been a very important, crucial part of the business. Ex: A lot of people say Jazz is the cornerstone of modern-day music. Ex: Data is the cornerstone of our business.