Expand and Contract
Clicking the 'Plus' (+) sign expands the folder line to show the next level down of folders and file types i.e. list all the folders and file types in the folder you have expanded. The 'Plus' sign will then become a 'Minus' (-) sign and clicking this will contract the expansion (i.e. hide the subfolders and file types). Double clicking a folder line expands the line in the same way and double clicking an expanded folder contracts it.