I kind of know what it is but I don't fully understand it. Wikipedia says,
the engine pressure ratio (EPR) is the total pressure ratio across a jet engine, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor.
I know where the entry of the compressor and exit of the propelling nozzle but I don't get what they mean by pressure. Do they mean air pressure?