It's a software development toolkit for creating applications
written in the flex language that run on the flash runtime.
Similarly there's a Java SDK for creating Java apps, ditto for
C/C++, groovy, ruby, and the list goes on and on...
SDKs generally include a compiler or interpreter, a set of
libraries that provide some generally useful capabilities (e.g.,
math operations, string operations), and possibly some useful tools
such as debuggers.