Reserved terms are the set of key words in a WordPress. You should avoid this terms in your plugin or theme files. Because it leads to conflict with core functionality. So don’t use this reserved terms in following places.
- Passing a term through a $_GET or $_POST array
- Registering a taxonomy or post type slug
- Handling query variables
To learn more about WordPress reserved terms, please see http://codex.wordpress.org/Function_Reference/register_taxonomy#Reserved_Terms