It should make it a lot easier to understand this tutorial is you are familiar with the data types involved in keyboard access, so I'll explain them first.
SDLKey is an enumerated type defined in SDL/include/SDL_keysym.h and detailed here. Each SDLKey symbol represents a key, SDLK_a corresponds to the 'a' key on a keyboard, SDLK_SPACE corresponds to the space bar, and so on.
SDLMod is an enumerated type, similar to SDLKey, however it enumerates keyboard modifiers (Control, Alt, Shift). The full list of modifier symbols is here. SDLMod values can be AND'd together to represent several modifiers.