Virtualbox guest additions allow better integration between the host and guest operating systems like seamless copy/paste, flexible screen resolution of the guest os etc.
After installing an OS inside VirtualBox you should install the Guest Additions right away to get the best features and functionality out of it.
It installs driver programs that enables native resolutions same as you current host OS, and also enables copy paste across the host and guest OS.
After installing the guest additions you can check if the guest additions are properly installed and active or not.
If your guest OS is a linux like Ubuntu or Fedora and you want to check if virtualbox guest additions are installed or not then you can use this simple command
$ lsmod | grep -i vbox
If the vbox modules are loaded they're installed and working. If nothing appears then probably virtualbox guest additions are not installed.
The output could be something like this for example
$ lsmod | grep -i vbox vboxvideo 12612 1 drm 264190 2 vboxvideo vboxsf 43659 0 vboxguest 214867 7 vboxsf
If the output of the lsmod command does not show vbox modules, then either guest additions were not installed properly or they are not loaded. Then you might need to debug further.