By load we mean cognitive load, the work your brain has to do the perform the task. And we thing about learning as changes in brain structure that represent new memories.
In the learning context, we think of load as having several factors. One is firing intensity. You neurons must fire to contract your muscles and more they fire, the harder the contraction. Another is novelty. Lifting a moderately heavy weight may require neurons to fire, but if you lift that weight often and easily, you may not learn or improve much. The other thing that adds to cognitive load is what else is going on in your brain at the same time.
So if you want to increase the results of your training adding a little load can help. I can come from adding resistance, speed, complexity, and novelty, You’ll learn more from trying to rub your tummy and pat you head at the same time than you will from a greater volume of independent rubbing and patting. Progressive overload (adding increasing resistance) works well to a point in weight training where recovery demands are to high. Additional progress might be made though periodization which essentially adds novelty to the workout. And in endurance sports, training at the same pace every day and just increasing distance limits learning and progress.