The equation f(x + y) = f(x) + f(y) describes a property of certain functions known as linearity. This means that if you add two inputs, x and y , the output of the function f for that sum is the same as adding the outputs of f for each input separately. Functions that satisfy this property are often called additive functions. A common example of such a function is the identity function, where f(x) = x . In this case, if you take any two numbers and add them, the function will return the same result as if you added the outputs of each number individually. This property is fundamental in various areas of mathematics, including calculus and linear algebra.