Guest account is the default account in Ubuntu is used for additional users who are not registered in the system, the Guest account has access to the desktop, using all the applications installed, but have limited access to system settings. if you think the account may not be necessary you can disable this option, proceed as follows :
sudo gedit /etc/lightdm/lightdm.conf
add following line in the bottom of lightdm.conf :
so it looks like this :
Now logout/restart your Ubuntu, that's it.
Step to remove guest account in Ubuntu