Any post made from the Tumblr iOS or Android apps uses the Neue Post Format (or NPF).
The NPF enables you to add multiple types of content all together, in one post! So, rather than a video post with a text caption, you can have videos, photos, text, audio, and more, all in one. The future is truly now.
In addition to mixing and matching content in your posts, NPF makes it so you can add images to reblogs and use new text styles (headers, lists, serifs, cursive, and typewriter) as well as color fonts. You can also drag paragraphs and images around to reorder them.
So, what does all this have to do with Themes, and why should you care?
Well, NPF posts may display differently than posts made from web (that’s because NPF posts can only be created in the mobile apps right now). Also, not all themes support every NPF feature.
To lessen the impact of this on blog themes, we created the "Use new post types" toggle.
This toggle adds a simple CSS patch to your blog’s theme to improve the way posts made from the mobile apps (aka posts using the Neue Post Format) appear on your blog. It’s on for everyone by default, but you can always turn it off from the “Edit theme” panel.