Interfaces between bulk media are often where critical molecular processes occur that can dictate the chemistry of an entire macroscopic system. Optical spectroscopy such as IR or Raman spectroscopy is often challenging to apply to interfaces due to contributions from bulk phases that dominate the spectra, masking any detail about the interfacial layer. Vibrational sum frequency generation (VSFG) spectroscopy is a nonlinear spectroscopy that allows vibrational spectra of molecules at interfaces to be directly measured. This Tutorial series is aimed at people entering the VSFG world without a rigorous formal background in optical physics or nonlinear spectroscopy. In this article, we present the fundamental theory of VSFG spectroscopy, with a focus on qualitative, intuitive explanation of the relevant physical phenomena, with minimal mathematics, to enable a newcomer to VSFG spectroscopy to quickly become conversant in the language and fundamental physics of the technique.