What's a Gusset?
A gusset is a triangular piece of fabric inserted in the side seam of sleeve and bodice to allow for more freedom of movement. It is the simplest way to correct a too small armhole.
Well, simplest from a construction standpoint. Not in the actual labor, because those things are incredibly fussy and although they work they're a pain to put in.
Why am I wailing about these oddly named bits of fabric? Because when I made the dresses for my YLP models, I did indeed make the armholes a wee bit too small (which shall now be corrected in all future dresses). It was, thankfully, the only alteration I needed to make, but of course it would be an incredibly fussy one.
Oh well. I'm thankful the problem was caught in the early stages so that I'll be able to correct the patterns for future dresses. Which is part of the whole point of these first four dresses.