Parent elements and resizing?

HTML-CSS
1

First off, what even is a parent element? tried looking it up online, and I’m not understanding.
I’m tryibng to build a survey page and keep getting an error message. Below are my code and the error message.

img {
max-width: 100%;
  display: block;
  height: auto;
  margin: 0 auto;
}
  1. The element should responsively resize, relative to the width of its parent element, without exceeding its original size.
    Try using the “max-width” style property : expected ‘none’ to not equal ‘none’
    AssertionError: Try using the “max-width” style property : expected ‘none’ to not equal ‘none’
3

the parent element is the one with the child element nested inside it
parent/child is a way to talk about nested elements. the child is nested inside the parent

4 
<header>
    <h1> This is the text </h1>
</header>

– In this block of code, header is the parent element of < h1> element and <h1> element is child of header element.
I hope this will give you a clear idea. If still confusion please feel free to write in comments.