To expand, state_dict is like a normal python dictionary. Default strict=True means that when the model loads, it will work if and only if the dictionary has keys with the exact same name as the parameters of the model AND nothing else.
With strict=False, you are saying that you don’t care if the parameters which are not included (by name) in the dictionary don’t get loaded and to ignore any keys which the module does not use.
strict=False is useful for example when you created a Module definition which only covers part of a previous module (think feature extractor made from a classification net), or when you want to expand a network and preload some of the weights from an existing module. In both cases, you need to be very careful to get the naming right or manually rename the keys in the dictionary.