The Kingdom of Jerusalem
Jesus was put to death in Jerusalem in Palestine. (Palestine was called The Holy Land). So Jerusalem was a holy place for pilgrims. The long journey there was full of danger - from bandits, disease, and shipwreck. But in 1071 the Turks, who were Moslems, conquered the Holy Land, and shut the Christian pilgrims out. The pilgrim route was closed.
The Pope was shocked. He urged Christian knights to go and fight a Crusade (or holy war) to win back Jerusalem from the Turks. He said that all who took part were sure to go to Heaven.
Hundreds of knights 'took the cross'. They made a big sacrifice. They had to leave their wives and families for at least two years. Many of them borrowed money to buy arms and horses, and to pay the wages of the foot-soldiers who came with them.
The knights met at Constantinople, then set out in 1097 across Asia Minor. The roads were bad and the food was poor. Most of the men were ill, and many died. But the Crusaders reached Antioch, and took it after a siege. Then they headed for Jerusalem.
The Turks fought hard to keep Jerusalem. (It is a holy city for Moslems too.) But in July 1099 the Crusaders broke in. They killed the Turks and destroyed the mosques (Moslem temples). Then some Crusaders went home. But others stayed in the Holy Land. They built churches and castles, and chose one of their leaders to be King of Jerusalem.
Walter Robson: Medieval Britain; Oxford University Press, 1991/2000, page 25 f.