Drafts

WordPress gives you the option to save a draft of your post. The term is quite explanatory. It means you don't have to publish your post right away, but you can still retain your progress and pick up right where you left off.

If you're working on a new post, the good news is that WordPress will save your progress automatically. Generally speaking, if you can see the Saved badge in the top-right corner of the editing panel (next screenshot), then it means that all your progress up to this point has been saved: 

You can safely navigate away from the page and not worry about anything getting lost. If there's no Saved badge, then there will be a link labeled Save Draft in its place. You can click on it to save your progress manually.