In general, signals consist of various kinds of frequency components. Frequency, expressed as Hz, means how many repetition of waveform, periodically generated, occurs in one second. If the periodic waveforms occur 10 times a second, the frequency is 10 Hz. If more waveform occurs in one second, it is called as high frequency signal. If less waveform occurs, it is called as low frequency signal.
Filtering in the signal processing means Frequency Filtering that filters specified frequency range as the analyst wants. So, if you use this method, you can get signals of specified frequency range as you want. This Frequency Filtering includes two methods; FFT (Fast Fourier Transform) and IIR (Infinite Impulse Response). Also, depending on the area subject to frequency filtering, there are Low Pass Filter, High Pass Filter, Band Pass Filter, and Notch Filtering.