A Burndown Chart visualizes how the sprint tasks that were planned at the beginning of the sprint are doing. The sprint tasks are what make up the sprint goal and they would have been estimated and debated during the sprint planning session.
Above ideal on a Burndown Chart means that the team is finding it difficult to complete enough tasks in the allotted time of the sprint so that all committed tasks will be completed. This can be a result of many thing such as:
Below ideal on a Burndown Chart means that the team HAS been able to complete the tasks faster than the team planned. This means that the team is creating features that pass the definition of done and also pass acceptance tests faster than they estimated in sprint planning. Ultimately this means the team can take on more user stories (assuming the user stories are done).
Oscillating around an ideal refers to a team that was able to estimate the user stories that they could complete in the sprint with pretty high accuracy.