For general purposes, hardening features should be described as reducing points of attack in a system. This can mean removing unnecessary code, or setting limits to attempts to use, login or manipulate a system. Hardening features are mostly configured based on the specific use case and known vulnerabilities.
Hardening Features for WordPress
For WordPress, the use case could differ for every configuration. For example: You can imagine adding WooCommerce (the use case) to your website adds a specific point of attack, mainly your subscribers and login page. Here you can enable hardening features like user enumeration, 2FA, minimising feedback and requiring strong passwords to limit the possibilities.