Jesus has become a sensation in Jerusalem and the people line the streets and hail him as the King of Israel. Their expectation is that he will deliver Israel from the Roman occupation. Jesus, by riding on donkey in fulfillment of Zechariah 9:9, was conveying the fact that he was a different kind of king than what the Jewish people expected. He was riding into Jerusalem in a humble manner towards his death by which he would bring peace.
Read John 12:12-19