"Collapsing the upper indentation with the bottom indent of the parent element" What is it?

Hi! I read on a website about the collapse of external padding and how to avoid them, but do not understand one thing:

the elements to which you apply the clear property to right, not top and collapses the indentation with the bottom indent of the parent element.

Source: htmlbook
And on habré:

Elements with the property cleared not collapse top margins bottom margins their parents.

Source: habr

How to understand this? Generally the top margin (margin-top) child element can collapse with the bottom margin (margin-bottom) of the parent element? Top child with the bottom parent? What does that mean?

Tell us how it is, if not difficult with html code where there is a collapse.

Or I somehow misunderstood this line?