Ambiguity is when something is unknown, uncertain or vague. With ADHD, we can easily get mentally paralyzed by all kinds of ambiguous situations, from not knowing where to find something to being unclear of what's expected of us. When you can't get started on something, if the words "I don't know..." come to mind, it's likely that ambiguity is the culprit.
Unfortunately, instead of identifying that there are unanswered questions or uncertainty getting in the way, many people with ADHD fall back on simply thinking they're overwhelmed. A state of overwhelm arises from feeling there's too much to handle -- which is an entirely different problem.
Why does it matter? Because the only way to break through the paralysis of ambiguity is to get the answers, clarity or direction you need. Strategies to overcome overwhelm won't help at all. When you can correctly identify what it is that you need to know, need to find or need to learn, you've taken a step towards getting the right kind of solution.