The Ecyclopaedia Metallum entry for Frostland Tapes has the necessary info:
To coincide with the band's 21st anniversary, this is a fascinating trip into the earliest years of the band and the first official release ever to contain all 4 of Darkthrone’s demos in one place.Collectors will no doubt leap at the chance to get a hold of this rare material, and rightly so. Darkthrone has almost never performed live in concert, which makes the included live recordings all the more intriguing. Some of the material present was previously available through the Limited Edition of Preparing for War, which was originally released in 2005. Still, there is enough new stuff here to merit obtaining Frostland Tapes, even if you are one of the lucky few to own a copy of the earlier compilation. It would have been nice if the two demos recorded during the Black Death era were included, but those have probably gone the way of London After Midnight.
Frostland Tapes also includes a rare live recording from Denmark in 1990 (one of only a handful of live appearances that Darkthrone has ever made) and, perhaps most significantly, the previously unreleased version of the infamous 1991 Goatlord rehearsal session.
These songs were intended to feature on the proposed second album, before the band changed direction and the tapes were confined to the vaults. Vocals were subsequently added in the mid-nineties, but they are presented here as the recording sounded in 1991.