What's a 'Hidden Light'?

From today's Roll20 Update announcement: 'An update decreasing the brightness of hidden lights in Updated Dynamic Lighting!'.

What's a hidden light as opposed to un unhidden one?

Hidden light usually refers to an object that emits light, much like touch or lantern etc. however its hidden in some way...usually via the object is left on the dynamic lighting layer. In reference to UDL I have not really tested it to its full extent...but its part doing the same thing you could in the legacy dynamic lighting thus getting closer and closer to parity and better performance moving forward.

Thanks, I thought it was something like that. Following on from that, why decrease the brightness?

I think it has to do with reaching parity with the old system, in the old system you could set the light on object to emit say 5 foot then set the dim to 5 or -5 and it would show only as dim light. Haven't realy tested in the new UDL but I suspect thats probably the idea.