When you squint, you squeeze your eyes almost all the way closed.
You might squint on a bright, sunny day, because there's too much light and it's hurting your eyes. Or, you might squint when you're confused, or when you're thinking very hard about something.
You might squint at something because you're trying to see it. What does it say here on my phone? Let me squint at it and see. Who's that over there? Let me squint at them and see.