The Xindi database stolen by Enterprise, at the outset of the starship's voyage in the Delphic Expanse, was one of several key intelligence prizes snatched by Starfleet during the search for the Xindi weapon. It contained comprehensive data about the Xindi, who were largely ciphers to both Starfleet and the Vulcan High Command at the time. Knowledge gained from the database heavily influenced Jonathan Archer's response to the Xindi threat.
Circumstances of retrieval
The database was obtained by an Osaarian merchant ship from a Xindi spaceship which they attacked. After the Enterprise encountered the pirates' hideout in a mysterious and ancient sphere, Ensign Hoshi Sato surmised that such a database was also in the pirates' possession, and would aid the Enterprise's search for the Xindi.
With the access codes having been acquired from an Osaarian pirate that Captain Archer was holding in the brig, the database was forcibly downloaded via a comm link by Sato, as Enterprise flew less than a kilometer away from the pirate ship. (ENT: "Anomaly")
Archer not only attempted to use this database in order to locate the Xindi weapon but also operated it to learn much about the Xindi themselves, including the different species of which that group was comprised. (ENT: "Anomaly") For instance, it was through this database that he first became aware of the Xindi-Arboreals, knowledge that later allowed him to make a positive identification of such individuals when he actually encountered them for the first time. (ENT: "Extinction", "The Shipment") The database also provided Xindi shuttle design specifications and the location of Rajiin's homeworld. (ENT: "Extinction", "Rajiin") However, other information proved more elusive, as Archer once spent hours merely trying to understand the way in which the Xindi organized their star charts. (ENT: "Extinction")
Analyzing sensor logs from the database enabled T'Pol to postulate the existence of a second sphere, much like one they had encountered when they had first obtained the database. (ENT: "Exile") The database thus allowed Enterprise to begin to uncover the existence of the whole network of spheres that created the Expanse. (ENT: "Chosen Realm")
After helping a religious order which had run into trouble, Enterprise was commandeered by the group's leader, Pri'Nam D'Jamat, who determined that Archer had desecrated the spheres, which were central to D'Jamat's religion. As punishment, the Pri'Nam deleted a significant portion of the Enterprise database, though it wasn't clear exactly how much of the Xindi database was affected by this purge. (ENT: "Chosen Realm") Nevertheless, Sato and T'Pol reconstructed about thirty percent of the lost data by the time Enterprise rendezvoused with Commander Shran. (ENT: "Proving Ground")
After an Andorian attempt to capture the Xindi prototype weapon, Shran secretly transmitted detailed scans of the weapon to Enterprise. (ENT: "Proving Ground") This more specific information – along with the fact that Archer tricked Degra into revealing the location of the weapon – presumably superseded the information in the database. (ENT: "Proving Ground", "Stratagem", et al.) As Archer formed alliances with the majority of Xindi species (ENT: "The Council", "Countdown"), the need for the database diminished further.