The "Jerusalem Cross" or "Pilgrims Cross" is actually the Emblem/Symbol of the Franciscan Order, that received from the Pope the Mission of "Costudia Terra Sanctae" - Guarding the Holy Land. They go back to the Monk and Saint Francis (Franciscus) from Assisi, of which it is said that one night he experienced the vision of Jesus so strong, that when he woke up in the morning, he actually had the "Signata" - the five wounds that Jesus suffered, on his own body. The 5 wounds are as follows: 2 on his hands (nailing to the cross) + 2 on his feet (also nailing to the cross) + one in the center of his body, the wound from his stabbing by Longinus, the Roman Centurion (Captain) responsible for the Executions on Golgota Hill. Longinus himself became a Saint, because he realised that Jesus was "Indeed, God's Living Son" and stabbed him in order to shorten his suffering.
The "Jerusalem Cross" is actually a representation of the five wounds Jesus endured, shown as five crosses, as they were positioned on Jesus' Body.
This is the main and common explanation. Others say it symbolizes Jesus & the 4 Evangelists or Jesus and the Four Corners of the World. During the Crusaders Time, this cross served as the Coat of Arms of the King of Jerusalem. It is also quite common for Catholic pilgrims from all over the world to have a tatoo of the "Jerusalem Cross" on their body. By doing this, they litterally have the "Pilgrims Cross" right under their skin!