How does exercise you body devolpe

An exercise strains the body and in response, your body heals/repairs itself and makes it better

example:
pushups strain the mucles in your arms and chest.
after the exercise, they might be sore. that's because they are damaged and need to heal. when they heal, they will be stronger and more durable

Efficiency is an action of doing something. for example when you take a test. good efficiency means you did well but bad effeciency or no effiency means you did bad.