save the value of a custom form in the database
hook_schema is called at install and unistall, so make sure your actually installing the module and not just turning it on.

If you've already had your module running on your site, switched it off at /admin/modules and then switched it back on, drupal doesn't recognize this as an uninstall/install event. Rather, you need to fully remove it from the DB by going to admin/modules/uninstall after switching the module off.