What is a child theme?

A child theme is a theme that inherits the functionality and styling of the parent theme.

What is it for and should I use it?

Child themes are recommended for modifying the code of an existing theme.
If you only need to add a few lines of CSS code then the Additional CSS box in the customizer will suffice.

However, if you need to make more extensive CSS changes or changes in PHP templates of the theme then using a child theme is highly recommended. It is an advanced tool and solid understanding of CSS and PHP is required to use it effectively.

But I could make the changes in original theme files as well, what’s the difference?

Yes, you could make changes in the original theme files, but once the theme is updated you will loose it all.
So, better safe than sorry;).