This means there exists an element $x \in I$ such that $y$ is a multiple of $x$ whenever $y \in I$; we say that $I$ is generated by $x$. Thus every element $x$ generates a unique principal ideal, the set of all left/right/two-sided multiples of $x$: $a x$, $x b$, or $a x b$ if we are talking about left/right/two-sided ideals in a ring. Clearly, every ideal $I$ is a join over all the principal ideals $P_x$ generated by the elements $x$ of $I$.