Working memory is the function of the brain that allows us to take newly acquired information and actively organize it in a way that makes sense so that our brains are able to then move it into long term memory.

ADHD brains, however, have shown to have pretty significant working memory issues. This has been well established across multiple studies. It's been further proven that ADHD brains tend to have more severe deficiencies in the Auditory component of working memory as opposed to the visual/spatial component. Meaning that ADHD brains tend to have a harder time processing language based instructions than they do visual.

An easy way to picture this is that ADHD brains are like a supercomputer with a very small amount of memory. So there are only a few things that can be processed at a time. If you add too much then something is bound to fall off.