Clicking the checkbox to the left of a todo will mark a todo as complete.
You can find the completed todos at the bottom of the todo board. Click Show completed todos to expand and view all completed todos.
Uncompleting a todo
You can bring back a completed todo by unchecking the box.
Note: If your todo is recurring, then a copy of the original todo will be created - and the due date will be updated to the next logical date based on the value you have chosen (Daily, Weekly, Monthly).
If your recurring todo has subtasks, all subtasks will be reset to uncomplete.