Hold information in mind while you use it.
Working memory is your mental scratchpad — the ability to hold a small amount of information active in your mind while actively using or manipulating it.
It's essential for mental math, following multi-step instructions, and holding onto the beginning of a sentence while you finish reading the end of it.
Working memory challenges are one of the more consistently studied aspects of ADHD — it can be harder to hold multiple pieces of information active at once, which is often why multi-step instructions or mental math can feel disproportionately effortful.
Being told a phone number and having to keep repeating it in your head just to walk to where you can write it down.