There are differences of opinion as to the exact details intended by the illustration, but the principle is that as followers of the Lord, we should be bold, yet wise. We should be careful and thoughtful, but also willing to take God-honoring risks. Any ministry we seek to engage in carries risks with it. We want to approach each ministry endeavor with wisdom and proper planning, but we must be willing to release control and trust the Lord in all we do.
In the ancient world, they would take grain and put it on a ship and let it set sail to be traded and make money. It was only by sending it off to be sold that it would be able to return as a profit. There were risks and dangers that would be ever present. It was possible the grain could be stolen, or the ship could wreck and be damaged. However, if a man wanted to gain a profit, he would have to be willing to cast his bread upon the water.
The spiritual principle for us is that while we want to be prayerful, thoughtful, and wise in all things when it comes to ministry if we desire to be useful and see Kingdom growth, we must be willing to take risks and realize the outcome of our ministry efforts are beyond our control.