Depends on where the file you are trying to include from is located.
Example:
/rootdir/pages/file.php
/someotherDir/index.php
If you wrote the following in index.php:
include('/rootdir/pages/file.php');
it would error becuase it would try to get:
/someotherDir/rootdir/pages/file.php
Which of course doesn't exist...
So you would have to use include('../rootdir/pages/file.php');