What is the Expect-CT http header?

Question

HTTP headers

How much I read, I did not understand anything about the new http header Expect-CT.
Can someone explain in simple terms why it is needed, what it is responsible for, how important it is and why reports are needed there?
For example, the site https://cdnjs.com/ has this:

expect-ct: max-age=604800, report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct"