In past versions of Safari, you had to navigate to a new tab to view its contents; however, with Safari 6 in Mountain Lion, Apple has added a new feature that allows you to preview all of the open tabs in a window.
When you create tabs in the browser either by clicking the plus button to the right of the toolbar or by pressing Command-T, you will now see a button appear next to the plus that looks like a small rectangle with margins. Clicking this (or selecting "Show All Tabs" in the View menu) will shrink the tabs' content slightly, displaying them side by side on a gray, textured background.
The current tab will be shown in the middle of the window with its title, URL, and content centralized, and the screen will show an edge of the adjacent tabs to the left or right of it. At the bottom of the screen you will see a series of dots that represent the number of available tabs, with the current one highlighted.
This new feature adds to the previous tab navigation options, which were directly clicking a tab or using the Shift-Command-Bracket hot-key combination for switching between them. With the new preview feature, you can scroll to the left or right using gestures or the arrow keys, or you can click the dots at the bottom of the window to go to their respective tabs.
While useful for previewing your various tabs, this view is not intended to be a full tab management system. You can close unwanted tabs by clicking the X in the tab's title; however, this is the extent of the management you can do from this view. There are no options for organizing the tabs, and you can't copy the tab's URL or otherwise interact with its contents.
The new tab preview is reminiscent of Apple's Expose, but without options or viewing the tabs in a grid and only showing the tab contents in line.
Despite these limitations, this new feature does offer a convenient way to pan through your tabs and visually select the page you wish to use.