If you have to stop writing a Journal entry before it's ready to be published, you can save your progress as a draft. The next time you tap the new Journal entry button, your draft will automatically be filled in.
However, you can only store one draft at a time. If you edit a draft and have to leave the Journal again before publishing, saving the draft will overwrite the old copy.
Your Journal draft is only saved on the device where you started writing; you cannot save a draft on your iPhone and resume writing it on your iPad, for example.
To delete a draft, either publish it to your Journal, or delete all of the text and tap the Save Draft button.