The Crusades can tell us a number of things about the changing face of Europe during the Middle Ages. It can tell us about such things as the importance of religion, the growing importance of the common classes of people, and the impact of economic growth. It can also tell us about the darker side of medieval Europe, the greed and bigotry that seem to be a part of the human condition.
Perhaps the most obvious thing the Crusades can tell us is that religion was a very important thing in Europe in the Middle Ages. The Crusades began when Pope Urban II called for an effort to help the Byzantine Empire and to retake the Holy Land from the Muslims. Huge numbers of people agreed to take part in this venture, largely because they felt it was their religious duty to do so. This is a clear indication of the importance of religion in those times.
We can also see that common people were becoming more important. It was not only the nobles and knights who went off on Crusades. Your text tells us, for example, about how Peter the Hermit was able to convince huge numbers of regular people to commit to crusading. The participation of common people in the Crusades shows us that this class of society was becoming more important and, in some ways, more assertive.
Thirdly, we can see that economic growth was occurring and that it was important in medieval Europe. Economic growth made the Crusades possible. If Europe had not been growing stronger economically, it would not have been able to support the Crusades. It would not have had the excess manpower to send on Crusades. It would not have had the money to support them. It would not have had the fleets of ships that could take them from places like Genoa and Venice to fight in the Holy Land. Thus, the Crusades show us that Europe was becoming richer.
Finally, the Crusades show us that people in medieval Europe could be evil. The main purpose of the Crusades was to fight Muslims for religious reasons. This is clearly a result of bigotry. The bigotry was not limited to Muslims, as shown by the fact that the Crusades brought about pogroms against the Jews and massacres of heretics. The greed of Crusaders led them to sack Constantinople, which was a Christian city that the Crusades were supposed to be helping.
Thus, the Crusades show us many things, both good and bad, about the changing face of Europe in the Middle Ages.