WordPress sites are designed using themes, or templates, along which the pages of a given site are built. Typically, the theme for the main page will set the tone for the themes for subordinate pages. This allows users of the site to experience a sense of continuity when navigating between different pages of the site.
When adding new pages to a site, rather than building the entire page from scratch, WordPress users can simply adopt the pre-existing theme and tweak it to fit their needs. This, however, can give rise to a new issue: if a change is made to the main page, users would normally then need to go in and make changes to all the sub-pages in order to preserve the sense of continuity.
A child theme makes it possible for a change made to the main page—the parent theme—to be duplicated across all the sub-pages. This saves time and reduces the chances for errors by vastly reducing the number of places a user must make changes.
A child theme will duplicate all the styles and features of the parent theme. In addition, if the parent theme contains any specialized functions, those will be reproduced in the child theme.