Something strenuous requires all your effort and strength, like a hard workout at the gym or carrying a backpack full of heavy books.
Strenuous can also describe something done with lots of energy or force. If your boss asks you to do a strenuous task like carry a couch up a flight of stairs by yourself, and you refuse by jumping up and down in protest, you have a strenuous objection to her request. To remember how to spell it, remember that something strenuous requires the strength of two of you! That's why there are two u's in strenUoUs.