A parent theme is the original, primary theme that a child theme inherits its design and functionality from. If you’re using a child theme, the parent theme is what’s actually supplying most of its templates and styling behind the scenes.
Both need to remain installed for a child theme setup to work — the parent theme provides the foundation, while the child theme adds or overrides specific parts of it.
Why This Setup Exists
The main advantage is safety during updates. If you edited a parent theme’s files directly, an update to that theme would overwrite your changes. Keeping customizations in a separate child theme protects them from being lost.
A Quick Reminder
Never edit a parent theme’s files directly if a child theme is in use — always make changes in the child theme instead. If you deactivate or delete the parent theme entirely, the child theme will stop working properly, since it depends on the parent to function.