How does redirection to pv actually work?

I am trying to understand how the redirection is exactly working in this command # tar -czf - ./Downloads/ | (pv -p --timer --rate --bytes > backup.tgz) What is the english translation ? All the data of tar is redirected as input to pv and then pv redirects it to backup.tgz ? **Then why is the bracket around pv required ?** Then how does this redirection make sense ? tar -czf - ./Documents/ | (pv -n > backup.tgz) 2>&1 | dialog --gauge "Progress" 10 70 After pv what is the meaning of 2>&1 ?