What it is
The cross is the most universally recognized symbol of Christianity. It comes in many forms — the Latin cross with its longer lower beam, the equal-armed Greek cross, the Celtic cross with its ring, the crucifix bearing the figure of Christ — but in every form it points to the same central event: Christ's atoning sacrifice.
Where it comes from
The cross was first a Roman instrument of execution. Crucifixion was brutal and humiliating, reserved for slaves, rebels, and the lowest classes; the cross was not an emblem anyone would have wished to display. For the first three centuries of the Church, the cross appears only rarely in Christian visual art — earlier Christians preferred symbols like the ichthys, the anchor, or the chi-rho. The cross began to be displayed openly as Christianity gained legal standing under Constantine in the fourth century, and grew over the following centuries into the central symbol of Christian art and devotion.
Theological weight
Whatever its visual form, the cross represents Christ's death on behalf of humanity — the central act of the Atonement, on which the whole of Christian theology rests. For most of the Christian world it is the mark of identification and remembrance.
The Latter-day Saint perspective
Latter-day Saints reverence the Atonement of Christ as the center of Christian faith, but generally do not display the cross prominently in worship spaces or personal devotion. The focus is on the resurrected and living Christ — the empty tomb rather than the cross is the primary symbol of Latter-day Saint Christology. This is a difference of emphasis rather than a denial of the cross's meaning; the Atonement, completed on the cross and finished in the resurrection, is everywhere central to Latter-day Saint teaching.
Why it's the SumBible mark for the D&C and Pearl of Great Price
The Doctrine and Covenants and the Pearl of Great Price are distinctively Latter-day Saint scriptures; their canonical reach is Restoration-era rather than ancient. The cross is the clearest universally-recognized Christian mark; using it for these volumes visually connects the Restoration canon to the broader Christian heritage in which it stands.