This is a filter removing high-frequency domain and passing only low-frequency domain with use of FFT algorithm, enabling pass of frequency domain below what you want. It is very useful in selecting low-frequency domain of signals and analyzing it.
Select data you want to apply this method in Analysis Window and click the icon of Low Pass FFT-Filtering in Analysis Tool Bar. This will display a dialogue box for frequency setting as shown in the below. Input frequency values as you want and click Apply button. Then, you can get results from pass of domain below the specified frequency in Display Window, which are registered at Analysis Window. Results after low-frequency pass filtering appear to move slowly as shown in the figure, so it is called as Slow Wave.