You squint when you squeeze your eyes almost shut. If you are trying to see something far away, you will probably squint while you strain your eyes to see more clearly.
When the sun shines brilliantly through the car windshield, the driver will squint, or squeeze her eyes almost shut against the bright light. This is probably a good moment for the driver to grab her sunglasses! You'll squint when a light is suddenly turned on in a dark room, and you might also squint when you're trying to read tiny type on a food label or medicine bottle. The expression on your face, and the action of doing this, are both called a squint as well. Squint comes from the now rare adverb asquint, or "from the corner of the eyes."