What does John 12:15 mean?

This is a quotation from Zechariah 9:9. Much of what happens during this triumphal entry is a fulfillment of prophecy. People in the crowd are shouting phrases from the Psalms (John 12:13). Jesus has purposefully selected a donkey—an animal associated with peaceful work—to ride into town (John 12:14). That not only coordinates with Scripture, it symbolizes the nature of His mission. Military leaders would have used a horse: an animal then used for war. Messiah's use of a donkey speaks to the fact that His kingdom was not, yet, to be "of this world" (John 18:36).

This deliberate action by Jesus is also a contrast to His earlier instructions. He often told people not to proclaim Him as Messiah (Matthew 16:20; Mark 8:30; Luke 5:14). This is in keeping with His frequent references to acting only according to God's timeline (John 2:4; 7:6; 12:23). All of this, also, maintains consistency to Old Testament prophecy. Daniel 9:25 predicts a certain time from a decree to rebuild Jerusalem's walls to the arrival of Messiah. The decree was March 5, 444 BC, and Daniel's prediction points to March 30, AD 33—which is the Monday prior to Passover (John 12:1, 12).

It's no exaggeration to say the Old Testament has predicted this very moment when referring to the Messiah. Sadly, Scripture also predicts that most people—even among those cheering—expect something from the Promised One very different from what's going to happen (Isaiah 53:3–6).