What does the dark triangle, located on the folder icons in Xcode 9, mean?

Folders without dark triangles exist as a group in your project and also (what is new in Xcode 9) exist as a folder in file structure.

Example:
After double-click in your project navigator you can see menu:

For New Group Xcode create group in project navigator and create folder in file structure (in finder) and of cource for New Group without Folder create only group.