Unlike the CSS background property "cover" which uses the center of an image as an anchor point when cropping, Smart Crop uses an algorithm to choose an anchor point based on the image itself.
This feature is an automated setting that works using colour / edge detection, and works best with images that have a clearly-defined subject.
To use it, enable the Smart Crop toggle on your image container in the Template Editor.
You can also set a custom Anchor to define where you'd like the subject to appear.
Head to Auto Crop Images Intelligently with Smart Crop ↗ to learn more about this.