Homing is putting the head at a precisely known position, normally done by moving the head so each axis touches an endstop. The center of the bed normally does not have an endstop.
Homing is necessary, because the motors that move the head around don't know at what positiion they are and the position can not be "meassured". The printer controller can only deduce the current position from the known "homed" position and all the movements added up. If you turn off the printer, then move the head around manually and turn the printer on again, the printer has no way of knowing the position of the head until it is homed again.