Someone please rephrase this for me?

In a perfectly inelastic collision, a bullet is fired into the stationary pendulum, which captures the bullet and absorbs its energy. The stationary pendulum now moves with a new velocity just after the collision. While not all of the energy from the bullet is transformed into kinetic energy for the pendulum (some is used as heat and deformation energy) the momentum of the system is conserved. By measuring the height of the pendulum’s swing, the potential energy of the pendulum when it stops can be measured. This allows the pendulum’s initial velocity to be calculated. Using the conservation of momentum, this allows the velocity of the bullet to be computed.