In the Parable of the Sower, Jesus Christ speaks of a seed being planted. If you've ever read Alma 32, you'll know that the seed is an important symbol of a testimony of the Savior that can grow within our heart. There were seeds that "fell by the way side, and the fowls came and devoured them up." There were seeds that "fell upon stony places, where they had not much earth: and forthwith they sprung up, because they had no deepness of earth. And when the sun was up, they were scorched; and because they had no root, they withered away." There were seeds that "fell among thorns; and the thorns sprung up and choked them." And there were many seeds that "fell into good ground and brought forth fruit, some an hundredfold, some sixtyfold, some thirtyfold."
In the Parable of the Wheat and Tares, the tares represent barren seeds of apostasy, while the wheat represents the fruits of the Kingdom of Heaven. Tares grow up to look very similar to wheat, but they cannot be harvested. They don't stop the regular wheat from growing.
I would like to draw a comparison between the thorns that choked some of the seeds in the parable of the sower and the tares in the second parable. The thorns stop the testimony from growing. I think of them as sins of omission. When we allow the world to get in the way of nourishing our testimonies, we aren't necessarily committing sins, but we are forgetting to keep the commandments that will keep the seed growing and strong. Tares are more like trial in our lives. Just because we have tares in our field, it doesn't mean that we are sinful or barren, it just means that we are going to have some hard times intermingles with the good times.
Interestingly, in Matthew 13:27-29, it says " The servants of the householder came and said unto him, Sir didst not thou sow good seed in thy field? from whence then hath it tares? He said unto them, an enemy hath done this. the servants aid unto him, Wilt thou then that we go and gather them up? But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them."
Heavenly Father knows that if our lives were perfect, they would be pointless. If someone were to take away all of our trials, our successes wouldn't be as strong. We would have a shallow earthly experience. We wouldn't have the opportunity to defend our testimony against the falsehoods and temptations that oppress. The wheat is stronger because of the tares. We are stronger because of our trials.