Explanation of halo reach?

You should of known this by now but Halo Reach is a prequel to Halo Combat Evolved. The story is set on PLanet Reach and your the new Noble Six after the previous Noble Six sacrified himself on the battle. If you watched the game, Noble Six gain a A.I that A.I. was cortana the chiefs computer Intelligence A.I.

Of course Noble Six Dies with honour since his line of duty is to protect reach and the A.I. Harsley entrusted for him to take to Keyes, which could then be untrusted with Master Chief.

So far no one knows if they return to reach, but most of the Spartens on Reach have died in the Fall Of Reach, and the only remaining Spartan II was Master Chief which is in the Pelican at the end in the Container. And after that should lead onto the stroy of Halo Combat Evolved.

Hope this has answered your question/

The end of Reach starts up Halo: Combat Evolved.

The ship did a "random" jump to escape Reach and ended up at Halo and started the story to the first Halo game.

The "alien information" is actually Cortana.

This would be the time to play through the all the other Halo games(from CE to 3) to understand the story.

remember that dude in the pelican?that was captain keyes and the end cutscene of reach is the begining of halo combat evolved