include externals locations?

In the subversion checkout dialog, there is an option to "exclude
externals locations". What does that mean?

I looked for this in the online help. I clicked on Version control /
Supported version control systems / Using Subversion Integration and
when I clicked on the subversion link, my machine only beeped. All of
the other links (cvs, perforce. starteam, vss) worked.

Cory