What does Matthew 21:18 mean?

But when the chief priests and the scribes saw the wonderful things that he did, and the children who were crying in the temple and saying, “Hosanna to the son of David!” they were indignant,

and said to him, “Do you hear what these are saying?” Jesus said to them, “Yes. Did you never read, ‘Out of the mouth of babes and nursing babies you have perfected praise?’”

He left them, and went out of the city to Bethany, and camped there.

Seeing a fig tree by the road, he came to it, and found nothing on it but leaves. He said to it, “Let there be no fruit from you forever!” Immediately the fig tree withered away.

When the disciples saw it, they marveled, saying, “How did the fig tree immediately wither away?”

Jesus answered them, “Most certainly I tell you, if you have faith, and don’t doubt, you will not only do what was done to the fig tree, but even if you told this mountain, ‘Be taken up and cast into the sea,’ it would be done.