This verse comes in the midst of chapter 19, which begins the weekly portion known as kedoshim (holy). Holiness is, among other things, about making distinctions. Separating clean from unclean, good from bad, male and female. The three specific commands immediately following "keep my decrees" are more examples of making distinctions. Don't mate different kinds of animals together. Don't plant your field with two kinds of seed. Don't wear clothing woven of two kinds of material. Animals, plants and clothing. Three areas of life we are to maintain holiness in. Cross breeding of species is not to be done. God made each animal according to its kind, so saith Genesis. Mating different animals together would make a new kind of animal. A mule is the sterile offspring of a horse and a donkey. Does this command against mating different kinds of animals together extend to not using the offspring of such prohibited mating? Would it be seen as giving acceptance or approval to violation of this command? Even if the person using the mule didn't actually mate the donkey and horse together to produce the mule? Perhaps because the offspring is sterile and thus cannot be fruitful and multiply, as God said to the animals on the fifth day (Genesis 1:22), is why God issued this command. That is sheer speculation on my part though. Sowing your field with different kinds of seed. Also, not to be done. We are not specifically told why. I do not know if a hybrid of different kinds of seed would be unable to reproduce naturally or would be inedible or what. Other speculations are welcome. And lastly, no clothing woven of different kinds of material. This is a general principle. Don't wear clothing woven of different kinds of material. What material? What constitutes a forbidden mixture? A specific case is given in Deuteronomy 22:11 which prohibits a mixture of wool and linen. We have here a combining of the previous two commands against forbidden mixtures of animals and plants. Wool comes from animals (sheep, usually). Linen comes from flax, a plant. Just as we are not to mate different kinds of animals together nor plant a field with different kinds of plants, so we are not to put on our bodies a woven mixture of plants and animals.