To begin, the Assets folder is the main folder. Think of it as the root directory, the parent folder, folder number one. The Assets folder contains the Assets (models, audio files, and so on) used in a Unity project. It will also contain any other folders, including the other special folders listed beneath Assets folder. Most API functions assume that everything is in the Assets folder, and therefore don't require it to be mentioned explicitly. This can be an oversight for game designers new to Unity, who may name the Assets folder differently. As you are likely to learn over time, there are some functions need to have the Assets folder, which is part of a file's pathname.  For example, certain functions in the AssetDatabase class require that the Assets folder is included in the pathname. As a result, it is better to include this from the beginning of your path to Unity and your project structure.