What effect does a 'margin: 0 auto;' have on an element in CSS?

The declaration 'margin: 0 auto;' centers an element horizontally within its parent container by automatically adjusting the left and right margins. The '0' sets the top and bottom margins to zero, while 'auto' allows the left and right margins to take up equal space, effectively centering the element. This technique is commonly used when the element has a defined width; without a specified width, the behavior may not work as expected. Using 'margin: 0 auto;' is a straightforward way to create a balanced layout in CSS, making it a popular choice among developers for centering block-level elements like divs.