Quite simply, tares are unwanted weeds and Jesus uses this parable to illustrate how evil (weeds) can exist alongside good (wheat) in the world. 

The Greek word used to describe the tares is zizania, which probably refers to darnel seeds – a type of degenerate wheat that is basically a weed. To the untrained eye, however it actually does resemble wheat, especially in its early stages of growth. Later on, as the wheat and tares come to maturity, the differences are quite apparent – causing one to all of a sudden wonder where these weeds came from!

One peculiarity of darnel seed is that it is frequently subject to a parasite fungus that renders it poisonous to both animals and man. Therefore, this parable, as understood by the agricultural society of the day, perhaps had far greater impact than it might upon us. Nonetheless, it stands as a continuous reminder for us to grow in a good place and watch the company we keep.