What is the meaning of Surjection?

A function for which every element of the codomain is mapped to by some element of the domain; (formally) Any function ${\displaystyle f:X\rightarrow Y}$ for which for every ${\displaystyle y\in Y}$, there is at least one ${\displaystyle x\in X}$ such that ${\displaystyle f(x)=y}$.

surjection

Source: wiktionary.org