The tasks section is where you can view and create tasks that must be completed for an entry before it can be published.
Click the Create new task link.
Enter what task you'd like to have completed.
Select a user to assign the task to from the drop-down menu.
Click the blue Create task button to finish.
The task will be listed in the Tasks section of the sidebar, and the entry cannot be published until it is marked as complete.
An assigned task
Meanwhile, the user you assigned the task to will receive an email notification. In the web app, a notification will appear over their user icon. Clicking the icon will open the user profile menu, where they can access their list of pending tasks.
This user has a pending task, as indicated by the orange notification