Let $S$ be a graded ring. In the notes of Ravi Vakil, he only says "define $I(Z) \subseteq S_+$", when $Z \subseteq Proj S$, where $S_+$ is all the elements in $S$ that are sum of positive degree homogenous elements in $S$.
He doesn't give the exact definition. Could someone please tell me how $I(Z)$ is defined? Thanks!