Google Apps Script works mainly within the Spreadsheets app to automate various processes. For example, users can automate the sending of e-mails based on data held in a spreadsheet, or create scripts that communicate with other Web services.
Why is it important? Because scripting allows customers customize and connect their information, which makes Google Apps a lot more stickier within an enterprise. If Google Search disappears tomorrow, people will easily switch to Yahoo or Microsoft. Once Google Apps, with scripts and storage, take hold in the IT space, replacing them would be almost impossible.