If something is uncomplicated, it's simple or obvious, like an uncomplicated art project for little kids or uncomplicated instructions from your teacher.
If something requires a lot of thought, planning, or many steps, it's complicated — intricate or confusing. Add the prefix un-, or "not," and you get the opposite, uncomplicated. You can use this adjective for anything you think of as "a piece of cake" or "easy as pie." Eating a piece of cake or pie is uncomplicated. Baking them? That's a bit more complicated.