Not able to understand what's do the statement

Always, I have a confuse/not understand file descriptors.

I know about the
0 for standard input
1 for standard output
2 for standard error
and 3 to 9 is a additional file descriptor but it's a theoritical knowledge.

Could you please give information about below two lines of code in AIX UNIX.

exec 3>&1
stdout=3 export stdout

Thanks in advance.

Just I would like clear one more thing.

This two lines are part of shell script function.

Thanks