This File menu command saves whatever changes have been made to the current document since the last time it was saved. If the document is being saved for the first time, the Save command prompts you for the location in which to save (see Save As). This command is enabled only if you’ve made changes in the document since the last time you saved it.
The keyboard shortcut for Save is Ctrl+S (Windows) or S (Mac).
Mac users: When first saving a document, Sketchpad suggests a filename ending with the extension .gsp. While Macintosh doesn’t require file extensions, naming Sketchpad documents with this extension makes it easier to share them with Microsoft Windows users or users on the Internet.