Blue Team's main mission here was to retrieve from what had been Halsey's lab on Reach 1) a lockbox marked by an Avar saber imprint, and 2) a set of three cryobins. They successfully do so, but are deliberately kept in the dark about their contents, aside from that they'll supposedly be a critical tool against the Created, and their own awareness that these cryobins generally store biological material. The safe bet is that they're probably cloned (and still living to some extent) brains of Halsey's, with which she'll create a Cortana 2.0, basically, who will in turn be the Offensive Bias to Cortana 1.0's Mendicant Bias. And, judging by
the dialogue that was found hidden in one of the trailers, possibly interact with some of Cortana 1.0's fragments that were cast off in Halo 4.
Aside from that, the Banished are on Reach to find a portal that leads to the Ark, overseen primarily by Escharum (apparently Atriox's mentor back in the day, the second-in-command of the Banished, wielder of a Gravity Axe, and a legendary figure in his own right),
Castor from Denning's prior novels (the Keepers of the One Freedom have been absorbed into the Banished), and a cunning ex-Silent Shadow Elite named Inslaan 'Gadogai. They likewise succeed in finding and activating the portal, and Atriox (plus several lieutenants) passes through it on a Lich. Atriox has left thousands of Banished back on the Ark, but is confident that they'll hold their gains there, and claims there are more pressing concerns he has to oversee personally in the Milky Way. In what is probably buildup for a HW2 followup, Castor then betrays Atriox, boards the Lich with a few dozen Keepers, and heads to the Ark with the intent of finally starting the Great Journey. Accompanying him is 'Gadogai, who's been effectively coerced into the Keepers by this point, and Veta Lopis + her Ferret team, who have actually infiltrated the Keepers and become some of Castor's most trusted operatives.