Child and descendant elements
A child element is an element within another element. The other element is the 'parent' element.
If a child element has an element within it, then the latter is a 'descendant'. This more general term refers to elements of any generation below the parent, including child elements. Other 'related' terms may also be seen, such as 'ancestors'.
An example of a parent/child relationship is when there is a 'div' within another 'div', as shown below:
<div> (parent element) <div> (child element) </div> (close child element) </div> (close parent element)