When the input signal (at frequency f0) of the amplifier is high, at the amplifier output you find not only the input frequency f0 but its harmonics also, 2*f0, 3*f0,.....
The amplifier shows a non linear behaviour, in first approximation you may write the output signal as a function of the input signal in this way:
y = A [ x + k2 * x^2 + k3 * x^3]
With this simple model you observe the compression phenomenon that may arise.
With 2 different frequencies (f1 & f2) at the input of the amplifier you 'll find other unwanted frequencies at the output such as (2*f1-f2) and (2*f2-f1).
These frequencies tones are called 3 order intermodulation products and they are very troublesome because they are very near to the input frequencies f1 & f2 and they can't be deleted with a filter.
You may quantify the intermodulation with the 3rd order intercept point (IP3) : the input power that give you the intercept between the Pout (f1) (in the linear behaviour) and the Pout (2f2-f1) (see the picture in the attachment).
In a Low Noise Amplifier the input signal is so small the you have not any non linear behaviour.