He spake also this parable; A certain [man] had a fig tree planted in his vineyard; and he came and sought fruit thereon, and found none.
Note 1 at Lu 13:6: It is important to look at this parable in its context. Jesus had just explained that the Jews who were killed by Herod and those on whom the wall fell weren't singled out for judgment because of any excessive sins that they had committed. Rather, it was just God's mercy that kept them all from being destroyed in a like manner. This parable is an illustration of what Jesus was saying.
The owner of this vineyard represents God (compare with Mt 21:33-43 and Lu 20:9-16). The fig tree represents the nation of Israel. The Lord planted the nation to bring forth the fruits of righteousness unto Him (Mt 7:16-20), but they had been barren. The whole nation was worthy of judgment ("cut it down," Lu 13:7) just as we would cut down an unproductive tree and replace it with a good one. However, Jesus, who is represented by the dresser of the vineyard, interceded for the nation and gained an extension of God's mercy to see if the nation would bring forth the fruits of repentance (Mt 3:8). They did not and therefore were "cut down" in A.D. 70 when the Roman army under Titus destroyed Jerusalem.
Jesus was illustrating by this parable that if it wasn't for God's mercy, they would have already been judged (see note 2 at Lu 13:2).