This term is generally used to imply interconnectedness between parts and that any change in one part (of a system) may affect another part. In natural and environmental systems, even with a relatively small action in one part, interconnectedness can produce a re-circulating chain-reaction effect and may in the end have an enormous impact on overall system behaviour. The impact is unpredictable, as a wide range of factors and conditions will be in play. This unpredictability and its corollary, difficulty in analysing and linking cause to effect, is the basis of what me mean by complexity.
Human activity systems, our social systems, display the same interconnectedness. Cause and effect of change can be equally difficult to link, not least because the key components of the system, human beings, have free will.