A Simulink model is made from Simulink blocks. The sum of all its Simulink blocks plus something else (configurations, parameters) becomes the Simulink model.
For simpler explaination, when you run a simulation of the model, the "InitFcn" of the model is executed first, then Simulink looks into all the blocks in the model to decide which block to execute first and which block to execute the next. If a block has an "InitFcn", then it is executed and goes to the next block. When all this "Initilization" is finished, the simulation begains, going step (in time) by step. In every time step, every block is executed at a particular order and this is repeated many times. All the "InitFcn" though, are executed only once per simulation during the "initizliaztion" period (sometime called compilation time).