$attachinfo shows your space used for attachments on disk vs what the db reports; this will only load if you are running a non-beta release, and you have debugging turned on. This behavior is handled by:
admin/dashboard.php - Look for line ~107-128
templates/default/admin/dashboard.tmpl
edit: the comment for the $attachinfo string within the dashboard.php file:
// Grab total attachments and size from the directory
// Because this process can be disk intensive, this option
// is only displayed when debugging is enabled.