Named after Alexander Graham Bell. Used to describe the effect of system devices on signal strength. For example, a cable has 6 dB loss or an amplifier has 15 dB of gain. This is very useful because system gains and losses can be calculated by adding and subtracting. A signal strength or power level.
Small signals are negative numbers.
This is useful since dB shows values in a logarithmic scale, not linear. The gain an antenna has over a dipole antenna at the same frequency. A dipole antenna is the smallest, least gain practical antenna that can be made. Then replace it with the antenna they are testing. Unfortunately, an isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical fade and System Operating Margins.