This option...

Does this...

New

Opens a new model and with shortcut "Ctrl+N".

Open...

Opens the open dialog from where you could browse to a model created earlier and open it. Its shortcut command is "Ctrl+O".

Save

Saves the currently open model and it could also be done with the command "Ctrl+S".

Save As...

Opens the save as dialog from where you could browse to the folder where you would like to save your model and then save it.