Fade Margin is an expression for how much margin - in dB - there is between the received signal strength level and the receiver sensitivity of the radio.
The figure below describes the term fade margin. Site A is transmitting with 33dBm (2W) power. After the distance to the site B, the signal level has dropped to -100dBm. This gives a margin of -10 dBm since the receiver sensitivity of the radio at site b is -110 dBm.
In very noisy environments, the level of the noise floor can be higher than the receiver sensitivity. (For example greater than -110 dBm in the above example). In this case, it does not help to increase the receiver sensitivity or using a higher gain receiving antenna. The only solution if the source of the noise can not be eliminated is then to increase the power of the transmission so that signal strength at the receiving radio is higher than the noise. However, in some cases, moving the receiving antenna can reduce the noise impact.