I do wish I had seen this and my efforts to find a re-run have so far proved fruitless but as I understand it they discussed the how's and wherefores of an alien invasion.
Specifically that the resource they would treasure most, above minerals and such would be more biological in nature.
From my point of view I'm left wondering why they would want to invade at all.
We terrestrially orientated humans tend to apply our perspectives and preconceptions onto the mind set of any potential alien species and quite incorrectly (in my opinion) assume they would want to inhabit our little blue ball at all.
I just don't see it myself.
I mean its just not logical. Surely a civilization sufficiently advanced to cross the vast expanses of interstellar space will not be bound by need to live under an uncertain sky but rather have adapted and indeed prefer to live within their 'ships' foraging for resources as they go on their way.
To that end, yes they may want some of our biological material (or possibly all of it) but would they actually want to live here? I very much doubt it, people climb to the tops of mountains, but they rarely choose to live there.
They might, at most, only have a few research teams posted on our planet prospecting for what they most need.
Ah now there's a worrying quandary. What if they then choose to scour our world of everything useful leaving an all but dead husk behind?