I didn't know about it either, tbh.
Consumers WORLDWIDE will be able to grab brand new material for a variety of RPGs --- no overstock, retail-priced or dead product here. The goal of Free RPG Day is to inspire gamers to play a new RPG, which will in turn, create sales through local game stores.
I think participating stores get a crate of RPG materials, which they can then distribute as they see fit (but for free, I think). If you go to a store, you can play demos, get RPG freebies, in the hope that you'll become a customer, which seems fair enough. Similar to free comic book day, or free book day. Just arbitrary days which are intended to give the industry a bit of attention.
I was also wondering if anyone had celebrated free RPG day before.