Rhythmic Complexity is a term referring to the number of varying widths that exist between attacks in a rhythm. For example, a rhythm of 3/2 has two levels of complexity: distance of 1 and a distance of 2. We might say the distance of 1 is a 1/16th note and 2 is an 1/8th note. Thus the rhythm would be 2 -- 1 - 1 - 2 -- or in differing beats it would look like this:

3: X-X-X-     
2: X--X--     

In the case of a 5/4 rhythm, there are four levels of complexity. It would look like 4 ---- 1 - 3---2--2--3---1-4---- or separately:

 5: X---X---X---X---X--- 
 4: X----X----X----X----  

So the distances between the X's are the different levels of complexity.

A rhythm of 7/4 has a whopping six levels of complexity: