SSH is short for Secure Shell and is basically a mechanism for securely connecting to remote shell on a host computer. You can do all sorts of funky stuff with SSH, but for the time being we are only interested in being able to get access to a bash shell on OMV.
On OMV, ssh can be found under Services->SSH. Tick the Enable box. By default, it works via port 22 and it is probably best to leave things that way. You can decide whether to allow root access via SSH, but generally this isn't required so leave the checkbox un-ticked.
Give User Access
Go to Access Rights Management->User and find your username. Double click to edit the user, and you should be presented with a dialogue. If you don't have a username set up, then you can quickly create one by hitting 'add'. Scroll through the list of groups until you find one called ssh and check it. You should now have SSH access.
Get an SSH client
To access OMV, you'll need an SSH client. If your client computer is a Mac or Linux machine, ssh is typically installed by default. So just open a terminal and type ssh <<username>>@<<server>>. You should be prompted for a password, which is the same as your regular Linux password (ie, the password you used when you set up the user in the first place).
On Windows, you'll probably want to download and install PuTTY.