When you transmit some information into space or channel there are some disturbances that rattles information signal acknowledged as “noise”. This noise is present in the channel/ atmosphere due to various factors like there’s thermal noise is due to the movement of electrons into the channel. This disturbance or noise is termed as Additive White Gaussian Noise.
Why Additive?
As noise will be combined to the transmitted signal y(t) = x(t) + n(t), where x(t) is the pioneer signal and y(t) is the biased signal with extra noise n(t) is being added to the signal.
Gaussian :
This Thermal Noise is spontaneous in behaviour, so this thermal noise is random in behaviour and possessing Gaussian distribution with 0 mean and variance as the Noise power.
White :
As white light consists of all colors we can see on prism after scattering, likewise, noise is equally distributed with the same power at all the frequencies. So, in frequency domain analysis, Noise level is uniform flat at every frequency.