WordPress Child theme is a theme that inherits the features, styling and functionality of another theme called parent theme. Child themes are the best ways to modify your WordPress theme without making any changes to the parent theme files.
If you modify a theme directly, your modifications may be lost when the theme gets updated. To resolve this problem, child theme introduced and rather modifying the core theme directly, you can simply override them with the templates in a child theme. When the parent theme gets updated, your modification will be preserved in a child theme and applied to the updated theme version as well.
Your theme customizations will be safe in child theme from future upgrades of parent theme because you only update parent theme and all customizations are stored and protected in child theme which you never upgrade.