Just like when you're working online, your changes will automatically be saved while working offline. However, in order for these changes to be saved to the server, you'll need to sign in to Google Docs with an internet connection from the same browser and same computer you used offline. Your changes will automatically be saved to the server when a network connection is detected by Google Docs.
You should sign in consistently to either http://docs.google.com or https://docs.google.com to install, sync and use Google Docs offline. If you install offline with one version (http), and then sync with another (https), the edits you made offline won't sync up until you sign in again with http.