help someone who can explain it to me

Each .main-section should have an id that matches the text of its first child, having any spaces in the child’s text replaced with underscores ( _ ) for the id’s

This is saying the element with class equal to main-section should have an id equal to the inner value(the text between the opening and closing tag) of the first nested child element with any space in the value replace with underscore
illustration

<section class = "main-section" id = "i_am_the_first_child" >
   <p>
        I am the first child
   </p>
   <p>
        I am the Second child
   </p>
</section>