Welcome to the world of VMware. In a nutshell, maintenance mode is the state you place a host into when you are doing maintenance tasks on the host. If you are going to be adding hardware, applying patches that require a reboot or just doing general troubleshooting of a host, you will place it into maintenance mode to evacuate virtual machines and stop the host from doing any tasks that would impact the virtual machines. While in maintenance mode, no virtual machines can be run on the host at all. In order for a host to go into maintenance mode, you need to shut down all virtual machines or migrate them to other hosts in the host cluster. This can be done automatically if you have a license for vMotion(live virtual machine migration) and the dynamic resource scheduler(DRS).
For more information, a good place to start is with the product documentation. VMware has that located at the following address.
VMware vSphere Documentation
Good luck and please feel free to keep asking questions.
Doug