Given two Functions $f: X \to Y$ and $g: Y \to Z$, we define their composition:

$g \circ f: X \to Z$

as $(g \circ f)(x) = g(f(x))$

Function composition satisfies Associativity so that $h \circ (g \circ f) = (h \circ g) \circ f := h \circ g \circ f$

More generally, we define the same thing for Morphisms/Arrows in a Category