There are 5 different task statuses available in MeisterTask: active, starred, completed, archived, and trashed. You can change the status of a task from within the task detail dialog.
By default, all tasks in MeisterTask's project boards are active until you complete, archive or trash them. The number listed underneath your user avatar i.e. next to your project boards is the number of active tasks currently assigned to you.
With the star button in the top bar of the task detail dialog you can star active tasks that are assigned to you. The star indicates that you're currently focusing on this particular task. Tasks marked with the star will show up in a special 'Focus' section right underneath those tasks that are due/overdue (and above the rest of your tasks).
When you're finished with a task, you can click the "Complete Task" button in the upper left corner to mark the task as completed. You will then be presented with additional infos about the task lifetime, and in project view the task tile will show a green checkmark.
You can keep your completed tasks in the project as long as you want. It's a great way to motivate yourself as well as keep an eye on your productivity.
If you've completed a task by mistake, you can easily make it active again. Just click on the three dots in the upper right corner of the task dialog and select 'Make active' from the dropdown menu.
Completed tasks can be archived by clicking on the "Archive" button, which appears in the dialog once the task is completed. Archived tasks automatically disappear from your active project board and are moved to the project's archive.
If you've completed and archived a task by mistake, you can bring it back anytime. To do this, switch to the project's archive, open the task in question, click on the three dots in the upper right corner and select 'Restore'.
If a task becomes invalid for whatever reason, you can trash it by opening the advanced menu in the upper right corner of the task detail dialog (three grey dots) and selecting 'Trash'.
If you've trashed a task by mistake, you can bring it back by switching to the project's trash, opening the task in question, clicking the three dots in the upper right corner and selecting 'Restore'.
In the top bar of your project board you'll find the view switcher, which lets you switch between your active, archived, and trashed tasks.