WordPress child themes are themes based on original theme. That is, child theme is a set of files which add some extended functionality to existing theme. Child theme’s files are placed in separate directory within WordPress wp-content directory. Obviously, child theme can not work if main theme files are missing.
Using child themes in one of methods for theme customization. It is possible to modify theme files directly, but this way there can be problems with restoring previous finctionality, if we have not backup.