I have a google cloud instance, I installed xfce, installed and configured VNC Server using this guide: https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-vnc-on-ubuntu-16-04
Now I'm in this step: Next, you may now use a VNC client to attempt a connection to the VNC server at localhost:5901. You'll be prompted to authenticate.
What's that VNC client and how do I connect it to the VNC server?