A thesis is the most important or foundational idea of an argument. If you write a paper with the central thesis that girls are yucky, you'll need to back that up with cooties-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.