The SDK is an abbr for Standard Development Kit. The RMXP SDK was created by several members, mainly scripters from rmxp.net, now hbgames.org. The SDK was made to compromize the compatability errors that 2 or more scripts could create when they where interfering the same classes in the script library's. Therefore the SDK chops much of the biggest default classes into smaller parts, so scripters can alias (add code to...) only that part of the class they need.
You proabbaly did not understand much from what I have stated above, so: The SDK, Standard Development Kit is a tool for scripters to decrease the change on incompatibilities with other scripts. When a script uses/requires the SDK, the user must have the SDK in his script list (best place: below Scene_Debug, above all other scripts).
The SDK can be found at hbgames.org in the sticky (scripts board), most recent version 1.5
Me™
p.s. so the SDK is just a script