Arise, heroes of men. For Ošinn All-Father gathers you to his hall, to quest and learn and to grow in strength and knowledge in preparation for Ragnarok, the end of days and doom of Gods. Seer and Hunter, Warrior and Poet and grim Berserk, all of worth are being gathered now into Valhalla.
Ragnarok is a one-year high fantasy LRP set in a mythic Scandinavia. Or more accurately, set in the afterlife of this. Players take the role of heroes, poets, or other such notable figures, who have died and been brought to Valhalla, where interactives are held. Various adventures and quests will be constantly taking place, some of which will be run in uptime as linears, some of which instead feature as downtimes.
The plot will be heavily based on Norse mythology, particularly focused on stuff drawn from the Eddas and so on. It will lead up to Ragnarok being run as the '3YGB', with players having the opportunity to affect major events (like the end of the world) but not avert them. Having a good working knowledge of Norse myth is likely helpful for plot prediction, but this is fine---the sources being used will exist IC in the game world.
The game will be run with a relatively simple global hits system. There will probably be progression. As the game is set in the afterlife, it is not necessarily going to be particularly realistic - injury will likely be relatively temporary, and abilities may explicitly refresh faster if they're being used awesomely. This is not intended to be a high PVP game---players are all working for Ošinn, and killing each other might be quite difficult as you're all (technically speaking) already dead. The odd option for playing Giant-cultists or other forms of traitor might exist, but this will not be standard.
It will probably be class-based, without multiclassing, and feature progression. Classes are going to be iconic archetypes of various forms: my current shortlist includes Warrior, Hunter, Berserker, Poet, and Seer. It will feature various forms of magic, but probably no point and click effects (at least in standard player powers). Players will not be able to play Ęsir, Vanir, dwarves, giants, or anything much along those lines, but may be able to acquire powers that aren't hugely available to standard humans.
In short, it's a game where you play mighty heroes fighting for the gods to prevent the world from being destroyed in the upcoming apocalypse.
Will a detailed knowledge of Norse myth or history be necessary?
In short, no. The game is set in a world inspired by history, but not directly drawn from it. There will be public setting information covering everything which one "needs to know" about both the mundane and mythic worlds.
However, if you choose to play a Seer or similar, it might be useful to do a little extracurricular reading to help your roleplay, and if you are deeply familiar with the Eddas you might find your plot prediction powers a little improved, because they will be used as 'sources of inspiration'.