Hi Khoi,
Node is the hypervizor which can run many guests (VMs).
We support a feature call Hosted-Engine, which allows the Node to run the Engine (the oVirt Manager) as a special type of highly-available VM on the Node(s), that it will manage.
(
https://www.ovirt.org/documentation/how-to/hosted-engine/ )
That Engine manages all Nodes that are added to it, and all VMs that they run.
In case e.g. Node1 fails you can simply start the VM on Node2, (as long as shared storage is used), in case of Hosted-Engine it will happen automatically - as long as you installed it on multiple nodes, which is the recommended deployment.
Hope that helped, if you have any further questions please do not hesitate to ask,