A privacy shield for a laptop computer is a folding cover that extends forward from the top and both sides of the screen, to prevent people sitting nearby from reading the data on your monitor display. As a side benefit, the shield also reduces or eliminates screen glare when you're using the computer outdoors on in bright artificial light.
When you shop for a privacy shield, make sure it fits over your computer's screen, and that it folds down to a size and shape that fits into your computer bag.