What exactly is Disk Cleanup going to delete with nothing selected?

Okay, So for some reason thumbnails have stopped appearing on video files larger than about 4.5 gigs, this is very recent and I'm not sure why, someone suggested to me that my thumbnail cache might be damaged or somesuch, and to run Disk Cleanup to reset the thumbnail cache.

Now that's all well and good, I actually haven't run it since I installed Windows 7 about 3 years ago. The thumbnail cache is at 100mb.

Now here's my issue, with nothing in the list selected, it says running the cleaner will delete 10 gigs from my C drive, so... what is this going to be deleting exactly? It's a little disconcerting to have nothing selected and have Windows telling me it's going to free up 10 gigs somehow.

Disk Cleanup keeps two totals of Disk Space. The first total is located at top of the menu, next to the window logo + disk icon. The second Tally is located under the tick sheet scroll window. One of them changes when you add or remove more ticks, the other is constant total, and is equal to the total if all ticks where chosen.