The basic idea of a function is that it is a rule for assigning one set, called the domain, with another set, called the range. Think of it in these terms. If you pick one object out of the domain, and plug it into your function, the function will spit out a single, unambiguous element of the range. As an example, think of a blender as a function. Pick any flavor ice cream and put it into the blender. Now let the function (blender) do its job. Out comes a milk shake. What flavor milk shake you get depends on the ice cream you put in (an element of the domain.)