The butterfly effect is an idea that is more commonly used in chaos theory. A small change can make much bigger changes happen; one small incident can have a big impact on the future. The idea started from weather prediction. Now the term is used for many things; some are scientific, and some are not.
The term butterfly effect comes from an analogy where a butterfly flaps its wings in Tokyo and a tornado occurs in Tennessee. Weather is hard to forecast, because small changes like this can have a big impact on the weather.
This term can be used in areas other than weather. Here a small change can also have a large impact. For example, it’s one reason why many people believe time travel to be dangerous. If a small change was made to history, that could have a very bad impact later on.