No such thing as a stupid question m8 (only answers - lol).
Essentially you're right, HSFX is a set of mods which change or add to certain aspects of the game - so you might have new aircraft, changes to flight models, corrections to existing aircraft, cockpit and map repaints etc etc which enhance playability or eye-candy, but you could also have mods which change internal parts of the game code such as updates to the QMB or FMB for mission builders.
These are only examples, but if you have a look on the developers website, you can get a break-down of all the mods within this update - be aware that it runs to over 4GB, so if you want to d/l it it may take a while.
Jabo