A combinator is a closed λ-term — no free variables. Every fold of computation can be built from a small handful of them. Smullyan named them after birds in To Mock a Mockingbird; we keep his names.
All these combinators are loaded as built-ins in the
visualizer — type them by name (S,
Y, theta…) and they expand to the diagrams above.
The page Learn covers how SKI alone is enough
to encode anything else.