|Libsamplerate - also known as Secret Rabbit Code (SRC) - is Erik de Castro Lopo's excellent audio resampling library. It was optionally available in Audacity up to and including version 2.1.0 if you compiled Audacity from source code.
Audacity 2.1.0 and earlier contained code to use Libsamplerate for resampling, but it was not enabled by default due to licensing issues. Instead, Audacity first used a library called libresample based on Julius Orion Smith's Resample algorithm, then from 2.0.3 to 2.1.0 offered Libsoxr as default resampling library, with both libsamplerate and libresample being compile-time options.
From Audacity 2.1.1 onwards, only libsoxr is offered.
Until 2.1.1, libsamplerate could be used if you compiled Audacity from source code at the command-line, and configured --with-libsampletate. On Windows building with Visual Studio, you could #define USE_LIBSAMPLERATE in win\configwin.h.
If you use libsamplerate, you should not install extra plug-ins in Audacity unless they are licensed under the GPL or some license compatible with it.