Let's assume that you are familiar with Unity and absolute file path (in an operating system concept) first.
Path "a/b/c" with type StreamingAssets is equal to Path.Combine(Application.streamingAssetsPath, "a/b/c") with type Absolute.
A general hint is, a absolute file path like "c:/some/file.ext" or "/Users/some/file.ext" do not work across systems (Windows/Mac form the editor and Android/iOS from the phone).
Choose what you want and set correct value.