What does "cornerstone" mean?

Native speaker’s answer
Rebecca
A "cornerstone" is an important quality or feature on which a particular thing depends or is based. It's similar in meaning to "foundation," "basis," and "keystone." Here, the speaker is saying that the Queen is the foundation of the Commonwealth, the figure upon which the organization depends upon. Ex: Quality products are the cornerstone to our business. Ex: The President is the cornerstone of our government.