A masterspool is the practice of printing your own spool out of filament, which will then be used to support your filament you purchase without a spool attached. The main idea is to create a reusable spool and create less waste.
(NOTE: I'm in no way affiliated with MatterHackers.com, nor am I an endorser of their products. There is also a version which Village Plastics has created.)
On MatterHackers.com website, they state:
Filament without a spool? Why are we making this? The short answer: because the community wants it. We had enough questions, comments and plenty of tweets asking if we had plans to pick up the Master Spool concept. Seeing the response and interest within the community made it clear to us: we needed to bring this idea to the States. With a joint effort between MatterHackers and Village Plastics, you can now purchase Master Spool refills from within the US.
They are tying to apply the Reduce, Reuse, Recycle mantra to create a cleaner environment for the rest of the world. While they are not the first to create or use a printable spool, they are pretty happy to be pressing forward with the idea of having a reusable spool and selling filament without a spool attached.
MatterHackers go on to state:
What are the benefits of the Master Spool? Not only is there the benefit of reducing plastic waste, using a Master Spool will also reduces shipping costs for new spools, and limits the clutter from amassing of a huge collection of used or empty spools. Rather than throwing away, trying to recycle dozens of spools, or trying to come up with a way to reuse them in some (like the Spool Tool), using the Master Spool means you can use all those filament scraps you have laying around on something useful and have one spool for all of your filament.
As far as where it started, it appears to have originated with this print on Thiniverse created by Dingoboy71. A well known 3D printer named RichRap created the reusable spool which MatterHacker promotes, though they say there are several which will work with their product (Village Plastic says pretty much the same).
If you get excited about saving the planet, then you should be excited about this. If you are a robust printer, going through tons (hopefully not literally) of filament per year, this method will save a lot of waste in the long run.
Realize there are (as of this writ) only limited suppliers of spool-less filament, though I think the trend for this type of product will increase in the future as the idea catches on. I guess time will tell.