The action of repeated additions: $a\times b=\underbrace{a+a+\cdots+a}_{\hbox{$b$ times}}$.
By looking at it from a geometric point of view, it is not difficult—although not trivial either—to see that $a\times b=b\times a$.
An interesting problem regards the complexity, or cost of multiplication. In 1960, Karatsuba improved on the "school method".