When you click on a Sprint to view it's progress, you'll presented with an overview that typically contains 3 primary elements; the burndown chart, your remaining tickets, and finally your completed tickets.
The burndown chart shows the current performance of your sprint, which calculates the ideal rate of work required (based on the amount of tickets, estimated effort and the overall duration of your sprint), and compares that to the current level of tickets completed:
As you can see above, in this sprint there are currently more tickets left to complete than should been completed already (by the estimated ideal velocity), so the sprint is currently behind schedule. Just compare the grey line representing the "ideal burndown" to the blue line showing what's been completed so far. The green line as an addition shows how many tickets remain in the sprint.
For more information about how to set estimated effort or time on a ticket, please see the following article.
You can see a list of any remaining tickets just below the chart, with the option to edit the backlog if you wish to bulk-update any tickets in the list or add any additional tickets:
Then finally a list of all completed tickets: