These are files containing source code for the Debian live images.
There are multiple different files here, corresponding to the different desktop environments..
Once you have downloaded all the ISO images you want, you will typically need to write them to media, either writeable DVD or a USB stick.
There are files here (SHA512SUMS, etc.) which contain checksums of the images. These checksum files are also signed - see the matching .sign files. Once you've downloaded an image, you can check:
For more information about how to do these steps, read the verification guide.
See the Debian CD FAQ for lots more information about Debian CDs and installation.
The images here were put together by the Debian CD team, using live-wrapper and other software.