When the browser window is smaller than the width of an element, that might happn when using laptop or mobile smartphone, the browser will add scrollbar or display the HTML element unclearly
Max-width will improve how the browser will display the HTML element.
In CSS we define the max-width in length values, like pixels (px), centimeters, or in percentage (%).
This element has a height of 250 pixels and a max-width of 900 pixels.
Resize your browser or open this page on your phone, to see the difference when using CSS max-width, Width of 900 pixels would not fit into mobile phone screen and will look unnatural, but max width makes sure the width will stop before 900px.