The details dropdown can be displayed be clicking the down arrow icon alongside the session that you want to find out more information about.
In addition to showing all of the data you can see in the Sessions panel, the details dropdown provides further information about that particular session and visitor. The details dropdown shows further visitor information such as language, operating system and browser, and number of opened tabs they viewed the property on during their visit.
Under Related sessions, you can see all of the visitor's other sessions on the respective property. In the image above, we can see that the user had seven other sessions on that property, while you can also view the sessions from the same IP address. If your account contains multiple properties, you can also view any sessions from that visitor on your other properties in this list.