Taking a punch is all about transfer of momentum. The momentum from the fist hitting you will be transferred to your body unless you do something about it. A fist travels with a specific velocity, v, towards you, with a mass, m. The product of the mass and velocity (mass times velocity) is the momentum (p = mv). When the fist hits your body, it slows or stops, but the momentum from the fist approaching you needs to go somewhere (m1v1 = m2v2 here, m1 and v1 are the mass and velocity of a fist, while m2 and v2 are the mass and velocity of the target of the punch). That somewhere is typically into your tender flesh.