There are times when a theme has to be modified to get it to look the way you want. For example you might want to make the header image smaller. These changes can be made by modifying a file.
The problem that arises is that your changes or wiped out when WordPress comes out with an update. So a child theme is created that is derived from the initial parent. you can think of the child theme as a group files the defining the page layout.
If you wanted to modify the header you would make a copy of the header.php file, modify this file then drop it into the child theme folder. WordPress checks this folder first and it doesn’t have to go somewhere else to look for the header.php file. Then uses this file to render the header.