How are these terms related? simple : complex?

The terms are related in the sense that they are antonyms. Antonyms are words that mean the opposite of one another. An antonym of 'slow' is 'fast', an antonym of 'thick' is 'thin'.

The definition of simple (basic, easy to understand, not complicated) is the opposite of the definition of complex (containing many facets, difficult to understand, complicated).