Folkvang is mentioned only twice in all of Old Norse literature: once in the poem Grímnismál (“The Song of the Hooded One”), and once in the Prose Edda. The relevant passage in the Prose Edda quotes the relevant stanza from Grímnismál, so we can assume that the Prose Edda used Grímnismál as its source.
According to Grímnismál, Freya takes half of the “weapon-dead” into Folkvang after they die. The other half are said to go to Valhalla, the more famous hall of the god Odin. The poem says nothing about what criteria are used to determine who goes to Folkvang and who goes to Valhalla. In any case, there are a number of other pieces of Old Norse literature that have contradictory things to say about who gets into Valhalla and on what basis. It seems likely that this stanza in Grímnismál is the work of a late effort at systematization rather than something the Vikings themselves would have necessarily believed.
The Old Norse sources say nothing about what Folkvang was like, or what the dead did while they were there. The Prose Edda mentions that Freya’s hall within Folkvang was called Sessrumnir (Old Norse Sessrúmnir, “Hall with Roomy Seats” or “Hall with Many Seats”). That’s a rather generic name for a hall, and the passage in question says nothing about it except that it’s “great and fair” – also a pretty generic description for a hall.
However, the idea that some of the dead go to Freya seems to have been reasonably well-established, although different sources indicate different conceptions of who went there upon death, and under what circumstances this occurred. Egil’s Saga, for example, has a world-weary female character declare that she’ll never taste food again until she dines with Freya. Suicide by starvation is obviously a very different kind of death than one that occurs in battle, so it’s impossible, at face value, to square this passage with what Grímnismál reports. All we can say with confidence is that Freya was indeed thought to sometimes take in some of the dead.
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 Simek, Rudolf. 1993. Dictionary of Northern Mythology. Trans. Angela Hall. p. 87.
 The Poetic Edda. Grímnismál, stanza 14.
 Simek, Rudolf. 1993. Dictionary of Northern Mythology. Trans. Angela Hall. p. 280.
 Snorri Sturluson. The Prose Edda. Gylfaginning, chapter 23.
 Egils Saga Skallagrímssonar, chapter 78.