From Halopedia, the Halo wiki
We Remember is the twentieth and final track (Disc 2 Track 13) included in Halo: Reach Original Soundtrack.
The track is a remix of Hymn for Reach (from the earlier track Epilogue), opening with a strings before being joined by vocals. Electric guitar then joins in, playing the main melody, while accompanied by a lively percussive beat on a drum kit. As the melody repeats, the electric guitar continues with variations of the melody before fading out. The track ends with the original string melody, but accompanied by the percussion.
We Remember is a bonus track found exclusively in the soundtrack, and thus cannot be heard in any part of Halo: Reach.
The name We Remember may be a reference to promotional material and tag line stating "Remember Reach”. It may also alludes to remembering the hard work and effort put into creating all Halo games. Incidentally, the title seems to continue the theme of memory, as soundtracks from Halo 2 and Halo 3 were titled Unforgotten, Remembrance, and Never Forget.