Phytochemicals is a term that simply means plant chemicals.
What has excited scientists is the fact that many of these chemicals, which had no known function in human nutrition, seem to have a variety of biological effects in the body. Many of them act as antioxidants, some modify the immune system, and others alter enzymes that metabolize drugs in our system. One good example is grapefruit. This contains an as yet unidentified chemical that alters an enzyme in the intestine which increases the metabolism of some drugs but decreases the metabolism of others. In other words, some drug levels will be increased and some decreased if they are washed down with grapefruit juice.