The engine mount is a vital part of the model rocket. It is the mounting made for the solid motor which would power the rocket. In launch vehicles, there is an assembled propulsion system for mounting the solid motors as well as the liquid boosters which make up the entire propulsion system of the rocket.
The engine mount consists of a circular cylinder, a metal engine hook and centering rings and bulkheads. The motor is pushed inside the circular cylinder and held in place by the metal hook. The centering rings are used when the cylinder is smaller in diameter than the body tube. The bulkheads prevent the motor from coming out of the mount.
[Source: apogeerockets.com]
The main purpose of the engine mount is to prevent the motor from back sliding and coming out of the body tube during burn off. to do this, the metal hook is inserted through the centering rings and held in place, thus, also holding the motor in place. Also the engine mount allows the rocket igniters to be pushed into the motor for ignition.