A visiting student a few weeks ago was talking to another student about some mathematics that I had never heard of before and I wrote down something that he wrote on the board.
$$f(z)=1, L=\mathcal{O},\qquad f(z)=z,L=\mathcal{O}(1),\qquad f(z)=z^2,L=\mathcal{O}(2)$$
Where can I read more about this? Sorry if this is vague, it's vague precisely because it is vague to me what this is. I believe these are related to complex line bundles, but I cannot find this $\mathcal{O}$ notation.
Daniel Huybrechts : Complex Geometry - An Introduction