Solipsism is the idea that only one’s own mind is certain to exist.
Very few people are sincere solipsists. Instead, solipsism is generally used as an argumentation tool. For example, one might ask how another knows that something is true, to which a scoffing reply might be “How does anyone know that anything is true? The only thing we can know for sure is that we exist.”
This point of view is related to René Descartes’ famous line: I think, therefore I am. Descartes wrote about his philosophy as if he were the first to ever do so…that is, he didn’t take into account any previous philosophical positions about truth when writing, and wanted to avoid making any assumptions when thinking about reality. This is the basis for modern philosophy (which I love, and studied in college) and a reasonable position to take when asking the question, “What do I really know to be true?”.
While solipsism shares Descartes’ radical skepticism, it goes beyond it to become irrational. While it’s true that I may be the only being to exist, I cannot live as if that’s true. I must live as if I am surrounded by others who also have minds, and that they are actual beings and not constructs of my mind.