Domain authority is a metric that estimates a site's strength in search results, based mainly on the quantity and quality of the backlinks pointing to it. Tools like Moz (Domain Authority), Ahrefs (Domain Rating) and Semrush (Authority Score) each calculate this metric with their own formula.
It is important to understand that domain authority is not an official Google metric. Google does not use any of these scores in its algorithm. But they are useful as a proxy: sites with many quality backlinks tend to have high domain authority and, likewise, to rank better on Google.
To build domain authority, the path is consistent link building over time. There is no quick shortcut. Each quality backlink contributes incrementally. New sites start with low authority and need months or years of work to reach competitive levels. What matters is that the links are genuine and come from relevant sources.