It duplicates the file descriptor 1.

Lets start with known examples.

You know what 2>&1 stands for. It redirects stderr to standard output. On those lines, are 3>&1.

Check out these links for posts on similiar lines.

exec command and field descriptors..
Printing output to the monitor

The bash manpage has those documented. Check the sub-topics under REDIRECTION.

Happy learning !

Vino