The FolderPath expression just repeats the Folder path parameter of file operations like Write text file in triggers like On file operation complete. You can get the name of the chosen folder after a folder picker completes using the FolderNameAt(0) expression. (I realized this purpose was not documented, so I updated the manual entry to cover that.)
You cannot get the full file path to the folder (C:\Users\username\Documents\somefolder). For privacy reasons browsers do not expose such paths, as it can leak personal details like your username. Besides even if you could get that full path, you can't do anything with it in the File System plugin, as it never uses full paths. You can only work starting from picked folders, or known folders or desktop, only ever knowing the relative path from that folder.