SNR (Single Number Rating (one number designation) indicates the number of decibels by which the noise level is reduced with the use of hearing protectors. The higher the SNR, the better you'll be protected.
For example, being exposed to a noise level of 110dB (when using a portable power tools) is an SNR protection of at least 30dB is required. 110dB - 30dB = 80dB. It is imperative to monitor the attenuation value with the SNR value stated on the packaging.