I don't understand what it means a child element nested inside

Tell us what’s happening:
what does it mean, “a child element nested inside?”
inside where? what does it mean by child…

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<p>Purr jump eat the grass rip the couch scratched sunbathe, shed everywhere rip the couch sleep in the sink fluffy furr catnip scratched.</p>
<p>Kitty ipsum dolor sit amet, shed everywhere shed everywhere stretching attack your ankles chase the red dot, hairball run catnip eat the grass sniff.</p>

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Challenge: Introduction to HTML5 Elements

Link to the challenge:

If you re-read the explanation, you’ll see that there is an example of nested elements.