Hargold, welcome to RP! Your scratch disk is just a place that Photoshop uses as a temporary working area to store interim files it is working on. If you have a large image with many layers at high resolution, PS will often not have enough RAM to keep everything in active memory, so it needs a scratch pad area. When you close PS down normally, it clears the scratch disk files.
Unless you designate a Scratch Disk in the preferences when you install PS, your system drive (C) will be the default scratch disk. If you have a 2nd physical drive in your computer, PS will run more efficiently if the other drive is designated as the scratch disk. The only time you will ever notice that there is real usage of the scratch disk is when PS is shut down abnormally (if PS crashes or the computer freezes while PS is active). Then you may find a file with the extension .pst. The file will be very large (usually several hundered MB and you will not be able to open it. You can just trash it.