Riding in all high and mighty on thunder strikes in golden armor filled with power metal and lightning-piss is all well and good, but what happens when the big boys take their hammers and dragons and go wreck someone else's shit? Who's gonna make sure to Realmtrams run on time and the Azyrite-coin tax is collected? Who makes all that Order happen?
The Freeguild and their friends does, that's who - and they make it happen with moustaches, black powder and sheer, order-powered grit - in other words, all the Empire models GW still makes until they decide to release newer, less not-Sigmar models.
Who are the Free Peoples?
Everybody who's not in an army on the side of Order, basically - Normal humans, Aelfs, Duardin and other races just living it up in the Realms, farming and tilling and shit. As Sigmar proclaimed that the next Age was for him and not for those edgy boys across the lawn, people moved from the reinforced Realm of Azyr into the other Realms, establishing everything from huge cosmopolitan cities to tiny hamlets, made possibly by the Eternals and their allies. Now, with some proper civilization happening, there had to be some people to protect all these fucking peasants from the horrors of the Realms.
The main people to do so is the Freeguild; the Imperial Guard to the Stormcast Eternals' Space Marines. Compromising of regular soldiers, the Freeguild beat up the nasties of the Realms and look spiffy while doing so. They also rebuild shit and make sure there's some stability to the weirdness of the Realms. They vary quite a lot; if you can imagine it, it probably exists somewhere. For example, the Freeguild that come from Hammerhal wear golden armor with blue clothes and closed helmets, while a Warhammer Community member showed off a hoplite-like army of Freeguild boys that looks like they'd fit in 300.
Apparently they also love big cat-birds, and cat-birds love them. The biggest ones get ridden by the generals with the biggest hats. There's an innuendo in there somewhere.