A burn down chart is a graphical representation of the work left to do versus the time.
If a burn down chart is created at for a two week Sprint having 8 Product Backlog Items (work) in the Sprint Backlog, the X-Axis will have 0 - 10 days and the Y-Axis will show 0 - 8 PBIs.
As the days pass and the work remaining reduces, the graph from the top left will gradually trend (burn) down towards bottom right.
The developers may (choose to) create and update the Sprint burn-down chart to graphically track the progress during the sprint.
The Scrum Master should neither create, nor update the Burn-Down chart, to ensure that the Developers retain the ownership of tracking and monitoring the progress of their Sprint work.