A decibel (dB) is the logarithmic unit in which PSNR is reported, and understanding it removes most of the mystery from a PSNR figure. PSNR converts the Mean Squared Error into decibels by taking ten times the base-10 logarithm of the squared peak pixel value divided by the error, compressing a huge range of raw error into a friendly two-digit number where higher is better. Because the scale is logarithmic, equal ratios of error become equal additive steps in dB: every halving of the error adds about 3 dB (ten times the log of two is roughly 3.01), every doubling subtracts about 3 dB, so a 6 dB gap is roughly a quarter of the error. That 3-dB-per-doubling rule is the most useful thing to carry. For 8-bit lossy video, PSNR typically lands between 30 and 50 dB, about 48 dB the finite ceiling short of a perfect match. The catch is that the tidy decibel number says nothing about where the error landed perceptually — PSNR's own limitation.
