XAudio2 filter radian frequency
That is simply converting analog frequency into digital frequency. For example, if you want a cutoff of 1kHz at a sample rate of 44.1kHz, the corresponding digital frequency is 2*pi*1000/44100 = 0.142... radiants.
In terms of DFT the digital frequency is the $ \omega $ in the $e^{^-j\cdot \omega }$ term of the DFT