GoDaddy has published a series of videos introducing basic domain concepts, including Domain Investing.
Basically, just like other types of investments, domain investing is buying the domain with the intention of selling it for profit.
However, the concept of domain in this video of GoDaddy is slightly different from my point of view. They say the domain is a digital address.
Personally, the domain is just the link to your website. Domain point to a digital address – called the IP address, which contains the content of the website you visit.