The following sentences is in Algebraic Geometry and Commutative Algebra by Bosch，page 442.
If $D$ is effecitve and $f_i \in O_X(U_i)$ for all $i \in I$, the sheaf $O_X(D)$ may be interpreted as the sheaf of meromorphic functions having "poles" of a type not worse than indicated by the divisor $D$. In particular, the unit section of $O_X$ then gives rise to a global section in $O_X(D)$ and, hence, to a monomorphism $O_X \hookrightarrow O_X(D)$.
My question is what's the unit section of $O_X$ he means? and how to get a monomorphism from this unit section?
Any hint is welcome, thanks!