The WordPress dashboard lets you edit plugin files in Plugins -> Editor. It also lets you edit theme template files in Themes -> Editor. You can remove the ability to edit these files in the dashboard.
Removing the ability to edit files in admin can help stop some hack attacks. Some WordPress sites are hacked by scripts that take advantage of this ability to edit sites from the WP admin.
To remove the ability to edit your files in the admin dashboard, place this in your wp-config.php file:
// Disable file editor in WP admin dashboard define('DISALLOW_FILE_EDIT', TRUE);