Child theme is the theme that inherits functionality as well as styling from the parent theme.Child theme is a better way to modify the existing parent theme.
Child themes are separate but have most of the functionalities of the parent theme.Wordpress first checks whether a functionality is present in child theme, if doesn’t it will use the parent theme.
Child theme is a great idea as it can modify and add to the functionality of the parent without modifying it directly.
If we directly modify the theme without creating the child theme, there are two choices:
1) Not opt to update the theme in the future or
2) Update and sacrifice the modifications you have made.
Advantages of using Child Theme:
1) If we are using the existing parent theme as itself and modifying it whenever we need to update the theme, our changes will be lost.
By using child theme, our modifications will preserved.
2) We can speedup Development time.
3) Also the developer can learn basics of WordPress theme development.
4) At any time parent theme acts as a backup (Easy to restore).