Freezing the model means producing a singular file containing information about the graph and checkpoint variables, but saving these hyperparameters as constants within the graph structure. This eliminates additional information saved in the checkpoint files such as the gradients at each point, which are included so that the model can be reloaded and resume training continued starting from where you left offa previous saved point. As this is not needed when serving a model purely for inference they are discarded in freezing. A frozen model is a file of the Google .pb file type.