The catalog product comparison feature is disabled by default.
To enable it, go to Products > Settings > Attributes. You will find the Comparison section below the attributes table:
To enable the comparison feature you will need a page where you want the product comparison to appear. If you don’t have such a page yet, go to Pages > Add New. As soon as you add a new page, go back to the comparison settings and select this page in the Comparison Page drop-down (the default is Comparison Disabled):
When the comparison page is selected, the Compare Products button shows up on the individual product page.
How the comparison works
The comparison table is added to the selected page automatically. You can also place it with the [catalog_comparison] shortcode.
The table compares two products side by side. For each product it shows the name, the product image, the short description and the product attributes. Visitors can change a compared product with the Select Products to compare drop-down in the table.