A couple years ago tabs were introduced in Internet web browsers such as Internet Explorer and Firefox. Basically tabs are where you can open different web pages at the same time within the same browser window.
If you look at the top of the browser window just above where the web page you are viewing appears you will see what looks like one or more tabs on the top of paper folders. These are the tabs. You can click on the various tabs and jump from one web page open to another.
To the right of the tab you are currently in will be an X and clicking on that X will close that tab. The first part of the tab gives the title of the page being viewed. This is the same information that appears at the top of the web browser in the different color line (often blue).
If you want to open another tab there is a little square tab on the end of the tabs you can click or you can do ctrl + t. To close all the tabs you can just close the browser (File, Close or X in top right corner of window). You will get a message that will ask if you want to close all the tabs. This is where many people first realize they have several tabs open. Ok closes all of them, Cancel will cause none to be closed.
Tabs are helpful in not needing to open many copies of your browser.