The meaning of the parable of the wheat and the tares is covered by click on the link (previous in this sentence). The meaning of the parable of the mustard seed compares the mustard seed to the kingdom of heaven. It says that the mustard seed is the least, or smallest of all the seeds, however, when it is full grown, it is the largest of all the herbs and birds come and sit in the branches. This alludes to the fact that the gospel of Jesus Christ (the seed) will start out very small, then eventually it will grow large and fill the earth. As with the parable of the mustard seed, the leaven also represents the gospel and how it is small and how it will then grow to become large.