A one-to-one function is a function of which the answers never repeat. For example, the function f(x) = x + 1 is a one-to-one function because it produces a different answer for every input. A function y = f(x) is one-one only when for no two values of x and y, we have f(x) equal to f(y). A normal function can actually have two different input values that can produce the same answer, whereas a one-to-one function does not.