SDK = Software Development Kit. In general terms an SDK is just the tools and libraries needed to develop for some piece of hardware or platform, or sometimes a set of tools that use an SDK to interface with a piece of hardware.
SSDK = "Switch Software Development Kit", an SDK for certain ethernet switches. Usually SSDK refers specifically to Qualcomm Atheros' drivers which communicate with their hardware
QCA-SSDK = Qualcom Atheros Switch Software Development Kit, which is a driver system for certain Qualcom Atheros chipsets.
I was a little unclear in my explanation. The nomenclature is unfortunate and misleading. QCA-SSDK is a Linux kernel module for controlling Qualcomm Athera switches through their ssdk interface and is needed for their hardware NAT among other things. It should very much be in scope for here, since it's part of OpenWrt.
No, I'm sorry, I don't actually know much about its use myself since I don't use Qualcomm Athera chipsets.