When saving a Rhino model, the objects are added incrementally into the file. If the file is being saved to a network location (i.e. a location that is not on the computer you are using, but rather a file server of some kind), this can be a very slow process. With the "Network Save" option, the file will be saved to your local machine first, in a temporary location, and then when the file is complete, it will be moved to the final file path. This reduces the number of transactions across the network and is significantly faster for models with a large number of objects.