I am a bit of a lighting noob- are mesh lights just meshes that have emissive lighting? And what is the point of adding a light behind the character if the renderer does not pick it up? Should I create these lights by scratch, or is there content available I could use for this purpose?
Can be yes, adding the light behind the character can highlight the shoulders, top of the head etc to help highlight them better.
What they mean by setting the opacity to the light to 0.00001 is that the light will still show in the render, but what is emitting the light will not be seen (if that is what you are asking). The back light will be a little high to get the top back of the head and shoulders.
In this image I have back lights to her right and left. You can create the lights yourself and there are ones available in the DAZ store (not sure if there are ones in the forums here as I don't check), I usually create my own and experiment with different setups.