NGFF stands for "Next Generation Form Factor". SSD is of course, "Solid State Drive". These drives are 22x42x1mm (aka "2242"). There are also NGFF SSDs that are much longer (2260,2280...) and those will not fit in the Chromebook. On the left is one I installed, on the right is the original HP part.
Note: I had problems with this 128GB drive.
ChromeOS somehow was corrupting the GPT and would not boot. I tried Chrome recovery and now this drive will not boot at all. I am trying to figure out how I can repair it. I think I will get an adapter so that I can connect it via USB and re-write the partition tables. I re-installed the OEM 16GB SSD and everything is working again using ChromeOS and Crouton/Saucy Salamander.