A device used routinely by Jesus in the Gospels in order to illustrate spiritual truths to his followers. In his use, it was a short story (albeit one without much of a plot) describing an everyday person doing an everyday activity, with expectable consequences. The object was to introduce new and deep ideas about God, salvation, and Jesus himself to the common, uneducated person using an allegory they could easily understand.

An example may be the best way to illustrate the device and its intent. From Matthew 13:

The kingdom of heaven is like a man who sowed good seed in his field. But while everyone was sleeping, his enemy came and sowed weeds among the wheat, and went away. When the wheat sprouted and formed heads, then the weeds also appeared.

The owner's servants came to him and said, "Sir, didn't you sow good seed in your field? Where then did the weeds come from?" "An enemy did this," he replied.

The servants asked him, "Do you want us to go and pull them up?" "No," he answered, "because while you are pulling the weeds, you may root up the wheat with them. Let both grow together until the harvest. At that time I will tell the harvesters: First collect the weeds and tie them in bundles to be burned; then gather the wheat and bring it into my barn."

Matthew 13:24-30, NIV

Jesus explained the meaning to his disciples shortly thereafter:
The one who sowed the good seed is the Son of Man. The field is the world, and the good seed stands for the sons of the kingdom. The weeds are the sons of the evil one, and the enemy who sows them is the devil. The harvest is the end of the age, and the harvesters are angels.

As the weeds are pulled up and burned in the fire, so it will be at the end of the age. The Son of Man will send out his angels, and they will weed out of his kingdom everything that causes sin and all who do evil. They will throw them into the fiery furnace, where there will be weeping and gnashing of teeth. Then the righteous will shine like the sun in the kingdom of their Father.

Matthew 13:37-43, NIV

The allegory to farming was something that everybody in that time could relate to; if you weren't involved in farming yourself, then someone close to you was. It had the secondary benefit of making the story easy to remember, so that it could be shared later with others.

If the Gospel accounts are taken literally, Jesus never explained his parables to his followers except for the twelve Apostles. He allowed each person to deduce the meaning from the story himself, encouraging thoughtful contemplation on the truth of what was said as well as discussion with others about what was meant.