What is the "decibel" used for?

The decibel (dB) is a logarithmic unit used to express the ratio between two values of a physical quantity, most commonly power or voltage. In amateur radio, it is frequently used to describe antenna gain, cable loss, or amplifier output by comparing the final signal to the initial signal. It does not measure a specific physical property on its own, but rather a relative change. This is why it is used to compare input versus output or a signal versus a reference level. This means distractors suggesting it measures frequency, sidebands, or waveform shapes on an oscilloscope are incorrect, as those describe distinct physical properties of a signal rather than a ratio.