There are multiple reports within Khan Academy that use the term attempts, and in some cases these refer to different actions. Below you'll find what this term means in each report.
In the Assignments tab, when sorted By student (as shown in the image below), Attempt indicates the number of times that each student completed the exercise. On this page the number in the colored box displays the best score for each assignment, so if a student completed more than one attempt, their best score will be shown.
The same is true of the Attempts column that appears in this view when you click on an individual student's name, shown below (e.g., Attempts is the number of times a student has completed an assignment).
This same meaning applies when you switch to the By Assignment view and click the fraction under the Completed column. A modal (a pop-out screen) will appear; in this modal Attempts column refers to how many times each student completed the exercise (see the image below).
As in the previous view, clicking a student's name in this view will show the same page, where number of attempts also means how many times the student completed the exercises (see the image below).
In these reports, we show In progress in the attempt column when a student starts the first problem of the exercise. Only after the student completes all of the problems in that exercise will the attempt be counted..
Students can always practice the exercise again after completing all the problems by revisiting the material and clicking Practice again at the end of the exercise. These will count as multiple attempts.
There is also a report that can be accessed on the Assignments tab that shows specific student data for for each problem within an exercise (see below), In this report, an Attempt indicates the completion of an individual problem. On this report we don’t display scores. The Last attempt doesn’t mean that this is the best attempt from the student--it is the student's most recent attempt.
For example, if a student got a problem wrong, they get to try again if they want by clicking Try again or Move on to skip the problem. If the student elects to try again, they are attempting the problem multiple times. This updates the Last attempt in the report page, but does not affect any of the other reports.
Note: When a teacher creates an assignment, we assume that, even if that student has completed the assignment previously, the teacher wants to student to complete this practice. With that in mind, we consider this particular assignment to be complete only after the student has completed the material after it has been assigned. That said, any work students do outside of assignment will still impact their mastery levels. If the same assignment is required by more that one class (e.g., by a core teacher and a remediation teacher), completion of the assignment will count for both. However, this means that work that a student does before the teacher assigned the material is not counted toward completion of the assignment, and will not be displayed in that assignment's reports.