A thesis is the most important or foundational idea of an argument. If the thesis of your paper is that chocolate ice cream is better than vanilla, you'll need to back that up with plenty of sundae-based research.
The noun thesis has more than one important sense to it. One definition of thesis is that it is the most important or foundational idea of an argument, presentation, or piece of writing. But it can also mean a large work of art, criticism, or scientific research that represents original research and is generally the final requirement for an academic degree.