If we would have assigned a different value for x, the equation would have given us another value for y. We could instead have assigned a value for y and solved the equation to find the matching value of x.
In our equation y=x+7, we have two variables, x and y. The variable which we assign the value we call the independent variable, and the other variable is the dependent variable, since it value depends on the independent variable. In our example above, x is the independent variable and y is the dependent variable.
A function is an equation that has only one answer for y for every x. A function assigns exactly one output to each input of a specified type.
It is common to name a function either f(x) or g(x) instead of y. f(2) means that we should find the value of our function when x equals 2.
