Ahrefs doesn't have a metric called 'Domain Authority'; that is a proprietary metric created by Moz. Instead, Ahrefs has a similar metric called 'Domain Rating' (DR). Domain Rating shows the strength of a website's backlink profile on a scale from 0 to 100.
It's based on the quantity and quality of external backlinks to a website. It's a relative metric, meaning a website with a higher DR generally has a stronger backlink profile.