The OED defines complexity as "the state or quality of being intricate or complicated." It's synonyms include intricacy, complication, ramification, convolution, elaboration, multiplicity, and and involvement.
The same dictionary defines simplicity as "the quality or condition of being easy to understand or do" and "the quality or condition of being plain or uncomplicated in form or design." It's synonyms include clearness, unity, chastity, classicality, singleness, straightforwardness, sophistication, and artlessness among several others.
Complexity and simplicity couldn't be further apart in meaning, yet everything is complex or simple depending on how you look at it. A shift in perspective is often just enough to help you see a problem differently.