A tall order is a task or job that is difficult to carry out. For a bodybuilder, lifting 500 pounds is a tall order.
Some things in life are easy to do. Some can be done with a moderate amount of effort or skill. Others are tall orders: a tall order is hard to accomplish, and it might even be impossible. Getting 100 on a test is a tall order. In sports, winning a championship is a tall order. Getting into a school like Harvard is a tall order. People often say "That's a tall order!" when someone else asks for something unrealistic.