Welcome to Android Central! Is it a Samsung tablet or an LG tablet? Is this the stock Browser, or Chrome, or some other 3rd party browser?
There is usually a tab manager icon at the top or bottom or the browser screen, looking like a rectangle, perhaps with a number in it. Tap it, and you should see your open tabs showing as smaller windows. You can usually swipe them away, or tap the X or - sign associated with each of them, to close them.