A Child theme is a theme in WordPress which inherits the features and functionality of a Particular theme. This particular theme is also called the parent theme. The child theme is also known as the sub theme of a parent theme. 

If you make any changes in the child theme, the parent theme will remain preserved. Also, if you make any changes in the parent theme, your child theme will remain safe from the updates. You can find child themes besides the themes in WordPress. Appearances >> Themes.  For making any changes or updates you need to activate it. You can make modifications in the child theme in the style sheet present in this folder.

The Child theme is a safer option when you need any specific changes on your WordPress website, you can just override the template you need. You can take advantage of the child theme. If you aren’t satisfied with your customization in the child theme, just disable the child theme and everything will be as it was before. Means WordPress will automatically start using parent theme.

Related Reading

How to Create Child Theme in WordPress : Tutorial for Beginners

What WordPress Theme is That : How to Check?