When you have ADHD, a great deal of work goes on inside your head that is invisible to other people.  I like to alert non-ADHD partners to this because it's too easy, since you can't see that brain spinning and working, to think your ADHD partner isn't doing anything.  Instead, lots is often going on.  Those with ADHD tell me that they might be:

• collecting their thoughts to make a more coherent argument, particularly when it's important they present themselves well (ex; in an argument with their partner)
• processing what another person just said
• keeping random thoughts and input at bay - trying to focus on the speaker
• trying to think through the steps it will take to do something, without admitting that is what they are doing
• dealing with difficult emotions that come up when they approach doing something that has not gone well before
• dealing with difficult emotions, such as fear, when thinking about doing something for someone they don't wish to disappoint

On these, last two, Jessica McCabe and her husband (both with ADHD) have created two great videos about the Wall of Awful that describe those emotional 'walls,' how they got there, and what to do about them.  I highly recommend them.

Wall of Awful video 1

Wall of Awful video 2