Aliasing is what occurs when a signal is not able to be completely reconstructed due to an insufficient sampling rate. This phenomenon may occur if the sampling frequency is below what is known as the Nyquist Frequency. The Nyquist Frequency can be expressed as: $ F = Fs*2 $ where Fs is the highest frequency in the signal.
To show some of the dangers of aliasing here are some examples.
Example 1: Say you have the signal $ x(t)= cos(2*pi*50t) $
While sampling above the Nyquist Frequency will eliminate aliasing it is possible to sample below it and still not get aliasing, however this depends on the signal itself and is not recommended for general sampling.
