Domain authority is the value of your domain which is calculated by the number of links your website s getting from high as well as low authoritative websites. The more higher DA websites link to your domain its likely your website DA will increase. Its a metric from MOZ. Also, it takes into account the relevancy of links pointing to all the pages of your website as a whole and calculate an average of all page links a domain is getting and then DA is decided. There is no specific formula from which you can calculate yourself. So you have to use MOZ tool or other tools in market.
I am really curious how you have 1,2k+ posts in the forum and you don't know what's DA... Also, you have a website about web design in your signature... I would do a quick research before asking such questions... Really curious why you didn't do this!
Domain authority is a evaluate of the power of a domain name and is one of many search engine position factors. Domain authority is based on three elements: Age, Popularity, and Size.
Domain authority - another stupid tool created by SEO "experts" to earn money from stupid people, pretending to be a very important thing. Don't bother yourself about it.