What does chkdsk do, exactly?

I think that the Microsoft definition is quite good, from your question-

"Creates and displays a status report for a disk based on the file system. Chkdsk also lists and corrects errors on the disk. Used without parameters, chkdsk displays the status of the disk in the current drive."

From what you have shown, your hard drive basically had a bit of corruption, but it looks like chkdsk was able to fix it.

This sort of thing unfortunately happens over time (bad shutdowns / power failures during writes etc.) and it isn't necessarily anything to be concerned about.

My 2 cents, mostly for future readers, considering the age of this thread.

Some users are undoubtedly unaware there are really two levels of chkdsk operation, selectable via command line switch: the slash F (chkdsk /f) and the slash R (chkdsk /r). Both instruct chkdsk to fix the errors it finds, but the difference is that slash F does NOT check the drive for bad sectors, whereas slash R does.

One way to think of it is: /F fixes software problems, /R software AND hardware, since bad sectors are actual physical damage to the disk surface. /R does essentially what the old SpinRite program used to do ("used to" because today's large capacity drives are for the most part beyond its abilities to deal with); analyzing every single sector on the drive, whether it contains data or is completely blank, and attempting to read AND relocate data it finds onto good sectors. Then it marks the bad sectors AS bad, so that Windows does NOT use them again, effectively "removing" the bad sectors from the system.

As might be expected, the /R option takes several hours to complete, so don't sit there staring at it waiting for it to finish. :)

I'm generally a huge critic of Microsoft, yet even I have to admit they got it right with this tool; used right it's easily worth the same as standalone programs that sell for anywhere from 40 to 100 dollars (on a separate note: the same can be said about 'defrag' which is also included with Windows; does as good and possibly a better job than commercial defraggers.)

Agree with Wil's answer in terms of what the big picture is. You probably don't need to worry what the error messages mean.

That said, if you what the error messages mean, you would need to understand how NTFS/FAT/filesystem of your choosing formats a disk. I think the Russinovich books from Microsoft Press discuss NTFS, and I'm sure there are other free web resources, eg this site seems like a decent NTFS discussion at first glance. But most likely this is more detail than you need to worry about.