Guest Additions are a set of drivers targeted for installation when an operating system has been fully installed and working. These drivers are used to optimize the guest system for better performance and usability. To install them, start the virtual machine, then access Tools from the Virtualbox menu bar and select Install Guest Additions. You will see that a virtual CD drive will be populated with these. An autorun menu will start. Simply click Next, Next, etc. until it's finished. Reboot the guest system and you'll have 3D graphics support, better mouse integration, USB support, keyboard/touch support, etc.
For more information, access the dedicated page created for this process: https://www.virtualbox.org/manual/ch04.html