Assets Folder Structure
The Assets folder contains a libray of individual objects for reuse in multiple environments. For instance this is where you will find the characters for your project or set dressing pieces. Generally importing files into UE follows two paths: Importing as an asset and importing as an assembly. Generally previs/art directors/specialty asset creators will be concerned with adding files to the assets folder and set designers will be more concerned with importing into the environments import folders. More on bringing files into UE here.
The assets folder is broken up into subcategories by film department and a few specially unreal type folders.
There is a detailed description of each folder in the linked file.
Of note are the textures and materials folders. The materials folder is a library of tilable material instances that you can apply to any object (be sure to make a copy of the material if you intend to edit it), and the textures folder contains generic textures for modifying mateterial instances such as adding a noise to your dust layer.
Each asset it found in its own folder with all unique files associated with it. Of particular note is that texture files have a tendancy to be duplicated repetidly and it is advantageous to avoid that fate. Linking to texture files in another asset folder is perfectly fine.