Alias concept in DSP
digital signal has a property that if any continous signal having freq Fm and it is sampled at sampling freq Fs, then digital signal contain interger multiple of Fs freq, K*Fs, K any +/- integer number.
That's why spectra of digital signal is multiple of Fs.
Now if Fs < 2*Fm, then spetra is overlapped at Fs/2 freq. Because of this overlapped spectra, information contained at near Fs/2 is loss. That means exact magnitude neat Fs/2 freq is loss. Lossing information is called alising.
To prevent alising, we have to pass signal through filter which has predifined response before sampling.