Solipsism is a theory or a view that one’s own mind is sure to exist, but anything outside of one’s mind is unsure, so the external world may not even exist.
It basically says that you may be the only conscious being in the whole universe and, by the way, the universe also doesn’t exist. You created it with your own mind. Everybody you see is just your imagination. In fact, you may even be sleeping and all you see is just a dream. As Morpheus famously said:
Have you ever had a dream, Neo, that you were so sure was real? What if you were unable to wake from that dream? How would you know the difference between the dream world and the real world?
Maybe we are on our way to a distant star and it’ll take us 90 years to reach it and we found a way to hibernate for 90 years and our life is just a dream we see while on our way?
And do you know what is the scariest part?
It can’t be disproved. It’s unfalsifiable.
Solipsism can be used for good purposes, like not caring what other people think, and also it can be used for very bad purposes.