If you happen to be editing the theme’s files with custom modifications you may be in a hurry when it’s time to update the theme. To avoid that inconvenience WordPress has a way of including changes through a “child theme”. A child theme is an almost-theme that references another theme, and that will only include those files which you want to modify.

From the Codex:

A WordPress child theme is a theme that inherits the functionality of another theme, called the parent theme. Child themes allow you to modify, or add to the functionality of that parent theme. A child theme is the safest and easiest way to modify an existing theme, whether you want to make a few tiny changes or extensive changes. Instead of modifying the theme files directly, you can create a child theme and override within.