The Ultimaker Cura Software Development Kit, or SDK, is a set of functions available to developers of Cura plugins. By using just the functions in the SDK, rather than any function available in the Python code base, you can ensure more stability across Ultimaker Cura releases.
The Cura SDK uses a semantic version scheme. This means that, in theory, only major updates to the SDK result in changes being needed in the code of a plugin. This depends entirely on which functionality a plugin offers.
More in-depth documentation about the SDK and it's versions is available on the Ultimaker Cura wiki.
Are you finished with developing and testing your plugin? Upload and submit it on contribute.ultimaker.com to get it reviewed by an Ultimaker developer and then publish it in the Ultimaker Marketplace!