An infrared window is like a special little door that lets us look at things that are too hot, without getting burned or breaking anything.
Normally, when we want to see something that's very hot, we have to open up a big door or take off a cover, which can be dangerous because the heat can burn us. But with an infrared window, we can keep that big door or cover closed and use the little window instead.
The window is made of a special material that can let infrared radiation (which is a type of heat energy that we can't see with our eyes) through without letting any other kinds of radiation or contaminants in. This means that we can use special cameras or equipment to look through the window and see what's happening inside without risking harm.
It's kind of like how you might look through a little hole in a fence to see the other side, except with an infrared window, we're looking through something transparent that protects us from heat and danger.