The Crusades were a series of religious conquest meant to spread Christianity and recapture the Holy Land (Jerusalem).
The Crusades were ultimately a failure, and the Christians began to become the ones being slaughtered. However, it is known as "history's most successful failure" because it had many positive effects on Western Europe.
The Crusades led to increased trade and travel. Western Europe came into contact with new places such as India, China, and the Middle East. This lead to Europeans having a wider world view.
European knowledge began to develop, and they were less ignorant towards the world. Even though the Crusades were ultimately a disaster, they lead to a period of exploration and knowledge.
Explorers like Marco Polo traveled to China and India. They helped expand European knowledge during this new age of exploration.
Without the Crusades, Western Europe would not be the same as it is today. It is even possible that Europe would have never come out of the Middle Ages!