In the "Drafts" tab you will find all of the forms which you have started to complete, but, instead of submitting the form you have saved the form to complete at a later time.
These forms are identified with a pencil icon. They also include a blue progress bar on the bottom to indicate how much of the form has been completed so far.
From this tab, you can choose to click on the form and continue editing/answering questions in the form. You also have the option to submit or delete the form directly from this screen.
To submit the form, press the form and swipe right and click "Submit":
To delete the draft, press the form and swipe left and click "Delete":
You will then see a confirmation screen to delete your form: