Domain Authority is a Moz.com metric meant to predict a domains ability to rank content on a search engine. It relies heavily on backlinks and anchor text just like Google. DA is a number between 0 to 100. It is increasingly harder to increase your domain authority. So a site may have 20 backlinks from 20 different sites and it earns a 10 DA but when that site has 40 domains linking to it its not going to have a 20 DA it might have a 15.
The Law Of Diminishing Returns is really easily seen in this topic. Over a 4 month period a DA33 site increased its linking domains from 400 to 800 and its DA increased to DA40. That was 7 points for double the domains. This site on 2/1/2019 has a DA41. A month ago this site had a DA40. In the last 30 days this site added about 34 domains linking to it for a total of 854. The other site previously mentioned had about 800 when it became a DA40 as well.
An indepth discuss is included in an article on the site. Domain Authority, Why Should I Care?