Welcome to the Aleph Sector 40k (universe) campaign. This is a sector wide "pick up and play" campaign which is largely narrative, but now has a number of specific theatres to cater for different campaign styles.
Sweeping conflict between Chaos and the Imperium in the Perseus Deeps is an ongoing and apparently endless struggle between the warriors of the Emperor and their hated traitors. The campaign theatre supports all 40k universe games including 40k, Kill Team, Battlefleet Gothic, AI and Epic. The main "campaign system" allows players to simply find an opponent, play the game then record the results with the GM - this generates "points", which the overall faction commander (or the GM if there isn't one), can spend on territory, strategic raids, fortifications or anything else feasible within the campaign. The larger the game - the more points, but the better your faction is doing gives you a bonus to your win. The Perseus Deeps is the main theatre of war in the sector, as it is close to the Cicatrix Maledictum and is key to the control of the sector.
More information on the "system" can be found here (you don't need to know any of this at all, but the GM (Duncan) works with faction commanders to make best use of their wins, and acts as an advisor, warning against strategically "unwise" moves - though is often ignored!)
Meanwhile in the Hadron Expanse, a new campaign theatre has opened up. The Tyranids, Orks and Necrons are seeking to pour through the Hecate Gap - a passageway through a set of warp storms that once sealed them off from the rest of the sector - while the imperium, chaos and Tau forces try to stop them, and the eldar try to seal off the region once and for all.
The whole map is however "live" and players who want to get into the narrative can drive their own plots and perhaps even trigger the next major theatre of war. Any and all games from matched play to one off narratives can be played - its as much as you personally want to make of it.
More information here.
And if you want background - there is a ton on this blog to delve into. The campaign has been running now since 2001, and the blog since 2006. In this campaign - there really is only war...