What is the use of assigning "let item" to a structural directive in angular

I am trying to understand the code found on internet link here. Its about structural directive in angular. I can't understand what this particular peace of code it doing (extract from AppComponent):

...
<li *listItem="let item">
{{item.header}}: {{item.content}}
</li>
...


The *listItem structural directive is assigned "let item" value. What does that mean? Can somebody shed some light on this please?

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