Hebb's rule is commonly summarized as "cells that fire together wire together." More specifically, Hebb's rule suggests that simultaneuos activation of neurons increases the bond between cells. The adage provides a neurological account of associative learning.
"When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased."
Hebb, D.O. (1949). The Organization of Behavior. New York: Wiley & Sons.