A function is defined as a relation between a set of inputs having one output each. In simple words, a function is a relationship between inputs where each input is related to exactly one output. Every function has a domain and codomain or range. 

Evaluating a function means to substitute a variable with its given number or expression. Given f(x) = 2x + 4, evaluate f(5). This question asks you to substitute 5 for x and simplify. To add functions, add their outputs; to subtract functions, subtract their outputs.