The True Cross was purported to be the actual cross upon which Christ was crucified, an enormously important relic in a time when the veneration of relics was a central tenet of Christian faith. Fragments of the True Cross abounded in medieval Europe, so much so that John Calvin was to famously complain that there was enough wood of the Cross in Europe to fill a ship.
The most important relic called the True Cross, however, was an almost-complete cross identified by Helena of Constantinople, mother of Constantine I, who found three crosses corresponding with the Biblical account of Christ's crucifixion (which states that Christ was crucified between two thieves) and, through a miracle, identified which one was the actual True Cross. This relic was enshrined in the Church of the Holy Sepulchre in Jerusalem.
By the time of the Second Crusade, this True Cross was a jeweled and gilded marvel carried before the armies of the Kingdom of Jerusalem to provide divine protection and inspiration to the troops. It was lost before the start of the Third Crusade, however, during the Battle of Hattin when the army of Guy of Lusignan was defeated in the field by Saladin. The fate of the relic is unknown, but it was likely broken up for the value of the gold and jewels that covered it.
There is an odd footnote to the story of this True Cross: After the Fifth Crusade was defeated at Damietta, Sultan Al-Kamil (nephew of Saladin) offered to return a fragment of the Cross to the surviving Crusaders as a gesture of goodwill, but unfortunately a search of his treasury failed to find the relic; it had been misplaced by the treasury staff as unimportant.