Circumstantial evidence is like putting together a puzzle. When we make a puzzle, we look at all the pieces and try to figure out how they fit together to make the picture. We don't always have all the pieces we need, but we can use the ones we have to make good guesses about what the whole picture looks like.
In the same way, when people are trying to solve a crime, they use different pieces of evidence to try to figure out what happened. Sometimes they have direct evidence, like a video of someone doing the crime. But other times, they might only have circumstantial evidence, which are things that make it look like someone might have done the crime, even if there's no direct proof.
For example, imagine that someone's bike is stolen from their front yard. The police might be able to interview neighbors who saw someone suspicious in the area around the same time the bike was taken. They might find footprints near the bike that match a certain type of shoe. They might even find tools or clothing in the suspect's garage that match the tools or clothing used by the person who stole the bike.
None of these pieces of evidence prove for sure that the suspect stole the bike. But all of them put together can create a pretty strong circumstantial case that the suspect might be the one who did it.
So, circumstantial evidence isn't as strong as direct evidence, which is like having a video of the suspect actually stealing the bike. But it can still be very useful in helping investigators figure out what happened, even if it doesn't give them all the answers they need.