Trying to understand the definition of function field $k(X)$ defined with limit ($X$ is an irreducible variety)

Question

Let $k$ be an algerbaically closed field and $X$ an irreducible variety. I want to understand the definition of the function field: $$ k(X) := \lim_{\substack{ \rightarrow \\ U \not = \emptyset, open }} O_X(U). $$

After searching internet, I still don't know what this "limit" means here... Any explanation would be appreciated. Thank you.