High and low frequencies are dependent on the application. A low frequency for wifi would be 2.4GHz, while a high frequency would be 5GHz. For human speech a low frequency is 300Hz, while a high frequency is 3000Hz.

A graph of a fft (Fast Fourier Transform) allows us to visualize different frequencies. This example is adapted from [Matlab's fft help][1]. The following figure shows the first 100 out of $2^{20}$ samples of a time signal with two frequencies. Note how it is difficult to see the 1Hz component in this figure.

![Time signal][2]

To see the frequency content we plot the spectrum as show in the following figure. Here we can clearly see the two frequencies - one at 1Hz and the other at 50Hz.

![Spectrum][3]

Here is the code I used to generate these plots.

    fs = 2^10;        %sample frequency in Hz
    T  = 1/fs;        %sample period in s
    L  = 2^20;        %signal length
    t  = (0:L-1) * T; %time vector
    
    A1 = 0.2; %amplitude of x1 (first signal)
    A2 = 1.0; %amplitude of x2 (second signal)
    f1 = 1;   %frequency of x1
    f2 = 50;  %frequency of x2
    
    x1 = A1*sin(2*pi*f1 * t); %sinusoid 1
    x2 = A2*sin(2*pi*f2 * t); %sinusoid 2
    y  = x1 + x2;
    
    %Plot signal
    figure;
    set(gcf,'Color','w'); %Make the figure background white
    plot(fs*t(1:100), y(1:100));
    set(gca,'Box','off'); %Axes on left and bottom only
    str = sprintf('Signal with %dHz and %dHz components',f1,f2);
    title(str);
    xlabel('time (milliseconds)');
    ylabel('Amplitude');
    
    %Calculate spectrum
    Y = fft(y)/L;
    ampY = 2*abs(Y(1:L/2+1));
    f = fs/2*linspace(0,1,L/2+1);
    i = L/fs * (max(f1,f2)) + 1; %show only part of the spectrum
    
    %Plot spectrum.
    figure;
    set(gcf,'Color','w');  %Make the figure background white
    plot(f(1:i), ampY(1:i));
    set(gca,'Box','off');  %Axes on left and bottom only
    title('Single-Sided Amplitude Spectrum of y(t)');
    xlabel('Frequency (Hz)');
    ylabel('|Y(f)|');


  [1]: http://www.mathworks.com/help/techdoc/ref/fft.html
  [2]: https://i.sstatic.net/0MIIu.png
  [3]: https://i.sstatic.net/kAqQ3.png