Like any other explanation given by the question mark icon in Google Analytics, very confusing. “As clear as mud” basically. So… might be interesting to talk about this a bit.
It basically means that a direct session is a session where the utm parameters haven’t been overwritten. This happens in two cases, mainly. When someone visits your website via a link from another website and afterwards visits your website again from another website, the source changes and gets updated by Google Analytics. When someone returns directly to your website after visiting your website via, lets say facebook for instance, the parameters aren’t overwritten and thus Google Analytics tracks another interaction with facebook. So you can’t facebook two times as a source, not just one time. A technical limitation some of us aren’t fully aware of.