In mathematics, Function is an higher order concept which is being taught to you in higher classes. Still it is defined as:
A relationffrom a setAto a setBis said to be a function if every element of setAhas one and only one image in setB. In other words, a functionfis a relation from a non-empty setAto another non-empty setB, such that the domain offisAand no two distinct ordered pairs infhave the same first element.
The functionffromAtoBis denoted byf:A→B. Here,Ais the domain andBis the co-domain off.