Complexity refers, in general terms, to interrelatedness of components of a system. Complexity increases with the “number of components in a system, number of relations between them, and uniqueness of those relations.” A complex problem then will challenge students to consider more factors, more sources of information, and subtle, differentiated relationships between these factors and information sources.
For more on complexity, see Kannampallil, Thomas G., Guido F. Schauer, Trevor Cohen, and Vimla L. Patel. "Considering complexity in healthcare systems." Journal of Biomedical Informatics 44, no. 6 (2011): 943-947.