Sprints are the backbone of Scrum, where ideas are turned into value.
They are fixed length events of one month or less to create consistency. A new Sprint starts immediately after the conclusion of the previous Sprint.
All the work necessary to achieve the Product Goal, including Sprint Planning, Daily Scrums, Sprint Review, and Sprint Retrospective, happen within Sprints.
During the Sprint:
No changes are made that would endanger the Sprint Goal;
Quality does not decrease;
The Product Backlog is refined as needed; and,
Scope may be clarified and renegotiated with the Product Owner as more is learned.
Sprints enable predictability by ensuring inspection and adaptation of progress toward a Product Goal at least every calendar month. When a Sprint's horizon is too long the Sprint Goal may become invalid, complexity may rise, and risk may increase. Shorter Sprints can be employed to generate more learning cycles and limit risk of cost and effort to a smaller time frame. Each Sprint may be considered a short project.
Various practices exist to forecast progress, like burn-downs, burn-ups, or cumulative flows. While proven useful, these do not replace the importance of empiricism. In complex environments, what will happen is unknown. Only what has already happened may be used for forward-looking decision making.
A Sprint could be cancelled if the Sprint Goal becomes obsolete. Only the Product Owner has the authority to cancel the Sprint.