The HTC 10 may be a newly released device, but that doesn't mean that it has to be a long wait for developers to receive the tools they need so they can start working with the phone. HTC understands this it seems and has just pushed out the needed tools for developers to begin work on the device for anything from apps to customized firmware, like ROMs for users to flash. The tools which HTC has now posted up to the HTC Dev website is the kernel source code, and there are three downloads available as of now which include the kernel source code for the unlocked HTC 10 model, the European model, and the Verizon Wireless model of the device.
Since the HTC 10 comes running on Android 6.0 Marshmallow the kernel source code is also for Marshmallow software, naturally. The files for each model device are 217MB in size, so not quite the amount one would see from a large version update for the main Android software, but big enough that you'll probably want to be connected to a local WiFi network before tapping the download button. That's also assuming that developers are going to be downloading the kernel source code from their device for use later instead of just from the PC in the first place, which should already be hooked up to the internet through WiFi or ethernet.
If you're planning to work with the kernel source you'll want to make sure and download the correct file as the wrong kernel could end up doing damage to your device. If you're a developer this is likely something you already know, of course. The kernel source is not something that regular users will be able to get anything from, but it does lead to rather cool stuff down the road once developers have had their time with it to create some cool things for the device. So even if you have an HTC 10 but can't make use of these files, the developers that can will now have what they need to start building something that you can make use of in the future. Right now these are the only three versions of the device that have official kernel source code available from HTC.