Sprint override allows you to limit any of our ticket based metrics to a specific sprint. With in the sprint dropdown all historical sprints will be shown, but ultimately you have 3 options:
Choose a specific sprint
Choose the Current Sprint - this will dynamically update over time and always display the currently active sprint
Choose the Last Sprint - this will dynamically update over time and always display the last ended/completed sprint
Using this setting on a number of metrics in a single dashboard allows you to build dashboards that can be useful in ceremonies like the Daily Stand Up (current sprint) or a Sprint Retrospective (last sprint).