Virtualbox Guest Additions are a way to spice up your Guest OSes so they they also behave as just the normal OS and do not face any limitations. Once you install the guest additions successfully (you can read our step-by-step Virtualbox Guest Addition install guide in case of any problems), you have the following new features at your disposal for your Guest OS:
Kernel-Module: Gives you access to more core components of the Linux kernel.
vfs-module: Gives you access to the folder sharing feature of Virtualbox. With the help of this feature you can share any folder on the host system for it to be accessed by the Guest system
timesync: Synchronizes the clock in your Guest OS with the hardware clock of the host system.
x11: Provides a versatile video driver called vboxdrv that allows for better performance and features such as dynamically adjusting the guest resolution when the VM window is resized , seamless mouse pointer integration with the host and support for higher and better screen resolutions on the Guest OS.