A synergistic effect is an effect seen when two or more substances combine to create an effect that is greater than either one of them could have manifested by itself.
An example of this is the "baking soda volcano" experiment that many of us performed in science class. In this the combined action of vinegar and baking soda combines to create an intensely bubbling eruption that is caused by chemical interaction. Neither the vinegar or baking soda is capable of producing such a dramatic effect by themselves. In medicine, synergistic effects are seen when one medication might be prescribed in conjunction with another to intensify the action of the second medication. A synergistic effect can also occur by accident with drug interactions.
For example, alcohol can intensify the effects of some medications.