Reading around forums and in products, I see descriptions and weights for a "fast pack" setup for tents. What does this mean and what is it used for?
"Fast pack" refers to setting up a tent with just the fly, tent poles, and usually a groundsheet. You leave the inner body behind. For example, here are the fast pack instructions for the Big Agnes Fly Creek UL1 tent, which is designed for this setup.
Fast packing saves a significant amount of weight, but makes you more vulnerable to insects and cold.
A similar lightweight shelter option is to use a tarp and your trekking poles.