Tasks can be completed in the checklist tab. At the top of the screen, above Operations checklists, you will see this section titled My Tasks.
This gives you an overview of all tasks assigned to you. To complete a task, you can click the checkbox to the right of the title, but you can also click the arrow to the right of the checkbox for more information.
This information page will show you any additional provided descriptions, everyone it is assigned to, when it is due, and who assigned it.
Keep in mind that if it is also assigned to others, they can complete the task and it will be removed from your task as well.
When you have completed your task, we can then click "mark as complete".