Directories are a kind of website which are used to store businesses and their information. Simply to say, it is the process of adding website to web directory. In telephone directory you can find any number by name. Just like that, you can find any business and information about business.
It is one of the off-page SEO techniques. To submit your website details like name, link and description of your website, you need to select the best directory. Your domain authority will increase and helps you to get more traffic from google search engine if you keep doing this regularly.