During the digitization process, the analog s/g needs to be sampled acct to Nyquist's criteria that states:
The sampling frequency, fs, must be greater than or equal to twice the highest modulating frequency, fm, for faithful reconstruction.
$fs \geq 2fm$
Aliasing takes place, if this condition is not satisfied.
It leads to fold over, i.e. high frequency component masks the low frequency component and vice-versa.
Aliasing can be avoided by either
1] Band limiting the analog s/g to FM.
2] Using a sampling frequency slightly higher than 2fm.