The people recognize Jesus as the king as he comes riding in on a donkey (look at Zechariah 9:9--go ahead and read the rest of the chapter as well to help understand what the people were expecting from the king and the Lord). Crowds gather, they throw their cloaks on the road (as had been done for King Jehu, (2 Kings 9:13).
Large numbers of followers begin to shout loudly their thanks to God for giving them this king.
But, not everyone is pleased.
When some in the crowd tried to get him to get his disciples to quiet down, he responded "Even if they were silent, the stones would cry out." He's reminding them what Habakkuk had said about the powerful who try to protect themselves from debtors (Habakkuk 2:6-11).