Explainability
Explainability is the ability to understand and describe why an AI system produced a particular result. Simple models are easy to explain; large neural networks are not, so researchers build tools to look inside them.
In one line, for a 12-year-old
Explainability is being able to answer "why did the AI decide that?"
An example
A loan refusal comes with the main reasons: income too low for the amount, and a short credit history.
Why it matters to people
When a decision affects you, you deserve reasons you can understand and contest. Québec's Law 25 gives people the right to be told when a decision about them was made solely by automated means, and to learn the main reasons.