Mat 21:18 Now in the morning as he returned into the city, he hungered. 19 And when he saw a fig tree in the way, he came to it, and found nothing thereon, but leaves only, and said unto it, Let no fruit grow on thee henceforward for ever. And presently the fig tree withered away.
First thoughts most have when reading this account, "Why would Jesus be so destructive so as to do away with this sad little fig tree?"
You have to go back a few verses and read about His turning over of the tables of the money changers in the temple. "My Father's house is to be a house of prayer and you have made it into a den of thieves!" Then, the people behind this abuse, the religious leaders, confront Jesus to deride Him and ask where He got the authority to do such things.
It was this slide of the Jewish religion from practicality and godliness into politics, power grabs, financial gain and ritual that occasioned Jesus to make this analogy with a fig tree.
The fig tree represents Israel. Its lack of fruit shows that the Jews are no longer following God. The withering of the tree shows that the thrust of ministry will soon move away from the Jews and toward the Gentiles.
Please read Romans 11 for further enlightenment on the matter.
Of course, there is a secondary application, which is that faith can do mighty things. It can move mountains and remove unproductive bushes.