There are tasks that only require one action or one step. When such a task is completed Complete Task will complete it. (Swipe from right to left, More -> Complete Task)
For tasks that require two or more steps, you can use Complete Step.
When you Complete a Step (swipe the task from left to right) the first step is completed and a new step is added to the list. The new step is an exact replica of the old one, so, any notes that you might have will also be carried over. The step number is incremented. (the step number is the small number in the bottom right corner)
This is the default way to complete tasks/steps as a lot of things will involve more than one step and instead of you having to manually create a new step, it's done automatically.