The Command Panel allows you to run a sequence of commands.
Use the / key to open and close the Command Panel.
After typing in one or more commands, click the "Run" button to execute them.
The fill command allows you to fill the world with multiple blocks at the same time.
The fill command works with both relative and absolute coordinates. Usually you will want to use the ~ symbol for relative coordinates.
You can run a sequence of commands to do more sophisticated building. You can copy and paste your sequences to another document (such as Word or Google Docs) in order to save them or share them with friends.
Here are a couple of models from the blockerzz game that you can try: