What is a Burndown Chart?
A burndown chart plots remaining work (story points, hours, or tasks) against time, falling toward zero as a sprint or release progresses. The ideal line shows the steady path; the actual line shows the truth. The gap between them is the earliest honest forecast you'll get.
Its sibling, the burnup chart, plots completed work rising toward a total line — better when scope keeps changing, because you can see the goalposts move. Burndown answers "will we finish?"; burnup also answers "did the target grow?"
Worked example
A sprint starts with 40 points. By day 6 of 10 the ideal line says 16 remaining; the actual line says 26 and has been flat for two days. The scrum master doesn't wait for the review to act — the flat line means something's blocked, and the daily standup finds it: everyone's waiting on one API contract.