He said that a man sowed some good seed in a field. Then some one came and sowed tares in the same field. The good seed was wheat.
So the wheat and tares were growing in the same field.
The servant wanted to root up the tares. The man said, "No; let them grow together until the harvest."
Why did Jesus tell this story? See what He says about it:—
"The field is the world. The good seed are the children of the kingdom. But the tares are the children of the wicked one. The harvest is the end of the world. The reapers are the angels."
The harvest is very near.
That is when Jesus comes in the clouds of heaven.
The reapers will take the children of the kingdom home to heaven.