The understanding I've generally heard, and which seems supported by the context, is that the fig tree symbolises the unfruitfulness of God's people. This is particularly apparent in that both Matthew and Mark record it as happening alongside Jesus casting out people trading in the temple (Luke records the cleansing of the temple but not the fig tree thing). It is then followed by Jesus telling a series of parables against the religious leaders. There may also be a relation to the parable of the barren fig tree earlier on in Luke 13.