Alright, so from what i've been told ( I could be wrong)
It starts between the events of Halo 2 and Halo 3 from a different perspective. There will be similar things since they are both from the halo universe, but it's a differnt location/ you aren't a 'super powered cyborg' just a human. (no sheilds, less huge weapons, easier to die) so it's supposed to be more stealthy.
However, the end of the game hasnt' been specified, so it could end midway between the events of Halo 3, or it could potentially go past the end of the story of Halo 3.
So as an example, think of ...like think of Zelda: Orcarina of Time and the next game (was that majoras mask? or just say twilight princess, pretend they came right after eachother)
So pretend Twilight Princess comes right after Orcarina story wise.
So that's 2 games
Now imagine a new game that takes place in between the two games from Zelda's perspective instead of link (new location, different powers)