From Wikipedia (basic primer statement):

Solipsism (/ˈsɒlɪpsɪzəm/; from Latin solus, meaning 'alone', and ipse, meaning 'self')[1] is the philosophical idea that only one's own mind is sure to exist. As an epistemological position, solipsism holds that knowledge of anything outside one's own mind is unsure; the external world and other mindscannot be known and might not exist outside the mind. As a metaphysical position, solipsism goes further to the conclusion that the world and other minds do not exist.

The extreme philosophical version of solipsism is the truth that a person can only be conscious of one's own consciousness, therefore their may never be a full truth of ANY other consciousness. Read as: everyone on reddit is a bit except you.

The reasonable version of solipsism can be simplified to: "If I didn't see it myself, how do I know it's true?" This is an extension of you only trust your own thinking and senses.

The flat earth disinfo capitalizes on this reasonable form of solipsism by asking: "How do you know if 'x' is actually 'y'? It's only ever been told to you." It's an abuse of the same term "skeptics" use to shut down arguments - "great claims require great evidence". Then they flood you with pseudo science and bs.

There's a life raft in all the muck though, as the universal laws of physics would not allow such a large object to remain flat while rotating and orbiting our Sun.