What is "dark" traffic in Google Analytics?

It’s supposed to help you determine which campaigns are succeeding and which sources of traffic are delivering the best bang for your buck.And it’s supposed to help you, personally, get the credit you deserve for devising strategies and tactics to deliver on your objectives.

The term Dark Traffic refers to a number of visits to the site that are as direct traffic when Google Analytics can't tell the source of a website visitor

Dark traffic” is the term given to data collected about website visitors that is mis-categorised by analytics programs like Google Analytics, as coming to the website directly rather than through another channel such as a search engine or social media post.