Construction of the Hilbert Scheme

Question

I am reading the book "Rational Curves on Algebraic Varieties" of János Kollár. Definition-Proposition 1.2, begin like this:

Let $g:Y\rightarrow Z$ be a projective morphism and $\mathcal{O}(1)$ a $g$-ample line bundle. I would like to know what is a "$g$-ample line bundle".

Thank you a lot!