What does center/cover stands for in background shorthand in CSS?

Question

I saw this YouTube video and at the time (25:27), they are using a shorthand for background image.

(link - https://www.youtube.com/watch?v=8KVrdL0VcAk&list=PLu0W_9lII9agiCUZYRsvtGTXdxkzPyItg&index=39 )

/* home class */

     .home::before {
     content: " ";
     background: url('../images/pizaa.png') no-repeat center center/cover;
} 

I get that url() is used to set image url. no-repeat is for background-repeat and center is for position, but what is center/cover here? If it is background position or size, then documentation have no such value for the above property.

Please help, thanks in advance.