The Common Core Shift of Rigor doesn't just mean "harder" or "trickier" but a balance of conceptual understanding, procedural skill and fluency, and application in math instruction.
Even if you understand this definition of Rigor, however, it can still be challenging to understand what it means for a real classroom setting. How much time should be spent on conceptual understanding? Which element of Rigor should you focus on first? Can one lesson address multiple elements of Rigor?