What does fly on the wall mean?

"Fly on the wall" is an idiom that means to be able to observe a situation without being seen or noticed. It is often used to describe someone who is eavesdropping or spying on others. The term can also be used more broadly to refer to someone who is simply present and observing something, but not actively participating. For example, someone might say that they were "a fly on the wall" at a meeting if they were present but did not speak or contribute. The term "fly on the wall" is often used in journalism to describe reporters who secretly record or observe conversations or events in order to get a story.