What does it mean to die Al kidish Hashem? I learned that Moshe Rabbeinu died but his death did not do anything, but Rabbi Akiva died and his death was a signiticant event? How to understand it?

Kiddush Hashem literally means, "to sanctify God's Name" [His Name= His appearance in the world], and is used in the context of an action which brings others closer to God, Torah and the Nation of Israel. When a religious Jew acts properly, morally and idealistically in front of other Jews, or even gentiles, and brings them to think: "this Judaism is a good thing", that's a Kiddush Hashem. When Rabbi Akiva or the Jews during the Christian Crusades, even preferred to die rather than renounce their Judaism, when Jews in Soviet Russia dared to practice their religion even at the risk of being sent to Siberia, that surely gets others to think: "Wow, this Judaism is significant! If it's worth suffering or even dying for, than surely it's worth living for! We should learn more about this Judaism". Similarly, when an Israeli soldier sacrifices his life to save other Jews, it also gets people thinking about our national unity and altruism, what the Holy Land is about, what's so important about the survival of the Jewish nation, etc., all of which are very central ideas in Judaism. In addition, it's not accurate to say that Moshe's death "didn't do anything", for when a tzaddik passes away, we stop our daily lives to eulagize him, and analyze and learn from his great activities.