When you finish working on a task you can mark it as done to get it out of your way, so that you can better focus on your open tasks. When you mark a task as done you can no longer track time against it. However, you can re-open tasks anytime as needed.
To mark a task as done simply click on the icon to the left of the task or click on "mark as done" in the task detail right panel:
To view your completed tasks inside a project, choose the option 'show closed tasks'