[ale] Select()

[charriglists at bellsouth.net (Calvin Harrigan)]

I'm taking a long overdue plunge back into linux C programming.  While 
reading the man page for select(), I came across a line that has me a 
bit confused.

http://linux.die.net/man/2/select

"nfds is the highest-numbered file descriptor in any of the three sets, 
plus 1."

What exactly are they referring to? What is highest-numbered file 
descriptor?  Are they referring to integer value stored in the file 
descriptor? Perhaps it's the number of file descriptors added to the 
set?  It's not very clear to me.  Any insight would be greatly appreciated.

Thanks