Spring-loaded folders is basically something that makes it easier to move/copy a file from one location to another. This is how it works. Let's say you want to copy a picture file from a CD to your Desktop Pictures folder. First you open the CD and grab the picture file. Then you drag the file to your HD icon and hold it there (don't let go of it). After a moment a window will open showing the contents of your HD. In this window, you then drag the picture file to the Library folder and hold it there. After a moment a window will open showing the contents of the Library folder. In this window you would drag the file to the Desktop Pictures folder. At this point you can either drop the file or wait for the Desktop Pictures folder to open and then drop the file. Either way will copy the file to the Desktop Pictures folder.

Once you do drop the file, all of the windows opened in this way except for the front one will automatically close. This is, using the example above, if I waited for the Desktop Pictures folder to open, then when I dropped the file in that folder the Library folder and HD windows would close leaving only the Desktop Pictures folder window open. Also note, that any windows that were open before you started dragging the file around will also remain open.

One final thing, I said that this is used to move and copy files. However, there is one other use for this that I just thought of. Let's say you wanted to use a specific program to open a file and it is neither linked to that program nor is the program in the Dock. You can also use the spring-loaded folders to drag the file to the program's location and drop it on the programs icon thereby opening the file in that program.