Example II.6.5.2 in Hartshorne (Part II)

Question

Let $A= k[x,y,z]/(xy-z^2)$ and set $X = \operatorname{Spec}A$. Let $Y$ be the prime divisor of $X$ corresponding to the ideal generated by $y,z$. The example shows that $\operatorname{Cl}(X)$ is generated by $1 \cdot Y \neq 0$ and $2 \cdot Y = 0$. In what sense is this to be interpreted that $X$ is generated by a ruling of $Y$?