Fólkvangr is a plain where the goddess Freya reigns and where half of those who die in battle end up, the other half going to the Valhalla of the god Odin.
Fólkvangr is attested in the Poetic Edda and the Edda of Snorri. According to the latter, Freya's manor, Sessrúmnir, is located there.
What's the difference between Folkvangr and Valhalla?
To explain the division of the Einherjar ("warriors fighting alone") between Odin and Freyja, some neo-pagan currents explain that the warriors devoted to Odin are those who dedicate their existence to war and battles, i.e. the offensive warriors. The warriors devoted to Freya are those who fight to protect their families, their clans and their property, the defensive warriors.