A periscope is a tool you use to see things that would otherwise be hidden or obstructed. If you've ever been stuck in a submarine, curious about what's going on above water, then you've needed a periscope.
You may have heard -scope used with other words that have to do with seeing things, as it means "a viewing instrument." You use a telescope to look at stars and a microscope to look at teeny, tiny cells. A periscope is a tool that helps you see around what is an otherwise blocked view. It's often shaped like an "L" and can help you keep an eye on when your teacher will be rounding the corner!