The 'em' unit in CSS is a relative length unit, which means its value is dependent on another value. Specifically, 1em is equal to the current font size of the element or the browser default. If you haven't set a font size anywhere on the page, then it would be equal to the browser default, which is typically 16px. So, by default 1em = 16px. If you were to set a different font size on an element, then 1em would be equal to that new value.