If your original video footage was taken with a video camera that shot in a 4:3 ratio, this is how the video will look when played back an a more modern 16:9 ratio TV screen.
Many camcorders before 2005 were 4:3 ratio , and anything filmed at home before 2000 will almost certainly be in 4:3 ratio.
You will have black borders each side of the picture and the picture will look square as you are used to viewing a wider format, even though it is a 4:3 ratio as is was filmed and as it should be.
This example shows how the picture will look if it is stretched. The 4:3 ratio picture is stretched to fill the black space either side of the screen.
While this will give you a bigger picture it will reduce quality and stretch peoples features. for example a beech ball will no longer look round it will be like an egg on its side.
Some people watch their video like this as it fills the screen but it will be stretched across its width.
This is another picture mode that may be selected on your television, often called Zoom.
This mode will not distort the Beach ball mentioned earlier, it will still be round, however it will be larger and loose some of its detail. You may also find you would clip the top and bottom of the ball off with the image zoomed in.
This zooms the image to fill the width, but also equally zooms (and cuts off) the top and bottom of the picture.