When installing Smoothwall's Cloud Products, there's a field called "tenantID".
Most customers won't need this and can safely leave it blank.
A "Multi Tenant" Smoothwall system is one where multiple organisations use the same physical system, or cluster of systems. If your organisation has its own Smoothwall, and you're not part of a bigger unit like a MAT or District, then it's highly likely you don't have, or need a tenant ID.
What if I leave it blank, and I shouldn't have?
Untenanted Filter users won't get filtered, so you'll notice this pretty quickly. It's also fairly easy to reapply a tenantID, so while you should take care to have the right information, it's not the end of the world if you experiment. Do not, however use a tenantID if you're unsure it's the right one - you could end up with policies from another organisation, or send your logs to the wrong place.
Where can I find my tenant ID?
On your Smoothwall system: System » Administration » Tenants
You will see the list of tenants, and on the right, a UUID. That UUID id the tenantId.