Chain of thought
Chain of thought is a model working through a problem in written intermediate steps before answering. Asking "think step by step" can improve answers on maths and logic; reasoning models now do this on their own.
In one line, for a 12-year-old
Chain of thought is the AI writing out its steps, like you do in math class.
An example
"A bus leaves at 3:40 and the trip takes 55 minutes. Will I make a 4:30 appointment? Work it out step by step."
Why it matters to people
Visible steps make errors easier to catch. But the written steps don't always reflect how the model really reached its answer, so they are not proof.