The Three Root Folders
Every project you create in Scrivener, no matter which project template you use, will have three root folders in the Binder: Draft (also called "Manuscript" in some templates), Research and Trash. These are called "root" folders because they each serve a special function in your project, and they cannot be deleted (though you can rename them if you wish). This article will cover the basics of these root folders, but for more detailed information, please see the Scrivener Manual (available from the Help menu) under §6.2 The Three Root Folders.
You'll notice that if you try to drag a non-text file into this folder, you'll encounter an error reading "Media files cannot be dragged into the Draft folder". This is because the Draft folder is meant as a space for your manuscript and nothing else, so it only supports text files. This makes it easier when it comes time to compile your manuscript outside of Scrivener, and also helps keep your content organized by distinguishing between "stuff that's going to appear in the final product (for now)" and "stuff that I need for reference".
We understand that some users are writing manuscripts where images are a central part of the experience, such as recipe books or manuals. The Draft folder does not restrict you from adding images into your text, so long as they are imported into an existing document and not into the Draft folder itself. You'll want to use the Insert > Image from File... menu command to insert an image into a text document (though please do see our article on embedding images for important information about this feature).
Anything that is not your manuscript but is pertinent to the writing process can be kept in here. These can include things like a PDF on the mechanics of sailing, a pie chart of Zodiac traits, notes on the magic system you're building, or pictures of actors that would play your characters in the movie adaptation. Really, you can use this folder however you'd like, and you can even create multiple subfolders within this folder to further divide your research.
Don't feel pressured to use this folder for everything outside of your manuscript, though. You can import any file type anywhere in the Binder that isn't the Draft folder, so you aren't restricted to this one Research folder. For example, one might wish to create an "Outtakes" folder in their project that contains scenes that they've decided to cut (but still might want to draw on the material later, hence why they're not in the Trash). They might not feel that outtakes really fit under the "research" umbrella, so they decide to keep this folder outside of the Research folder, at the Binder root.
This is where ideas go to die.
Okay, not really. It's actually just a folder containing documents that you no longer want to see in other parts of your Binder. Because writing is such a fluid process, we wanted to include an intermediate place between "included" and "deleted", so you can let those documents sit a bit before you decide to get rid of them for good. Trash won't ever empty unless you do it yourself (Project ▸ Empty Trash...) -- when you do empty it, though, those documents are gone (unless of course you recover them from a backup copy) so be sure you're not making any rash decisions. If you don't want to empty the trash all in one go, you can simply right-click on a document inside it and choose "Delete" (you'll notice this option doesn't appear for documents that are not in Trash, so be sure to move them there first even if you know you want to get rid of them for good).
While there is no limit to how much you can keep in your trash, we do recommend going through it every once in a while and getting rid of stuff that you're certain you won't need anymore. This helps keep your project lightweight so that Scrivener can perform at its best when you're working on it.