A "theory" (lacking evidence, it should be called a myth) of the Earth's shape that is incongruent with the common understanding of the Earth and other planets in the universe, is unfounded, and contrary to gravity and basic laws of physics.
Flat Earth is believed mostly by conspiratards. It asserts that the Earth is a flat disc.
People who believe in this defend their assertions by calling any factual evidence of the Earth's spherical shape fake, and by using biblical verses and psuedoscience to contrive an argument.
We really live on a flatearth. NASA's satellite images are just fake to promote the evil researchers' NWO agenda.
I have no idea how a satellite works
I have never been on an airplane
I slept through 8th grade science class