What's the even ratio in rocket ?

A even ratio is the ratio between the area of the throat and the area of the nozzle exit, a nozzle's job is to convert high pressure into high speed exhaust gas the faster the engine throws propellant out the back the better the combustion has very high pressure and to convert that high pressure into thrust, you need to choke the flow until the exhaust gas becomes super sonic, and in order to exhuast up anymore you need to expand the nozzle.