When you use a web browser, you probably have multiple websites open at one time. All those open browser windows are a pain to track, but using tabs helps organize those windows. Instead of multiple windows open and scattered around on your screen, you have one window open, and tabs across the top that you can click to get to the window you need.
Safari isn’t the only app where you would have multiple windows open, but those other apps don’t usually offer tabs. However, macOS Sierra will bring tabs to apps that allow for multiple windows. Tabs will be in Apple apps built into Sierra, such as Mail, Maps, and TextEdit. It will be available in Apple’s iWork apps (Keynote, Numbers, and Pages). And it will work with third-party document-based apps.