|Libsamplerate - also known as Secret Rabbit Code (SRC) - is Erik de Castro Lopo's excellent audio resampling library. It is optionally available in Audacity if you compile Audacity from source code.
Audacity contains code to use Libsamplerate for resampling, but this is not enabled by default due to licensing issues. Instead, Audacity releases use a library called libresample, based on Julius Orion Smith's Resample algorithm.
Libsamplerate can be used if you compile Audacity from source code at the command line, and configure --with-libsampletate. On Windows building with Visual Studio, you can #define USE_LIBSAMPLERATE in win/configwin.h. See Developing On Windows for details.
If you use libsamplerate, you should not install extra plug-ins in Audacity unless they are licensed under the GPL or some licence compatible with it.