An expression like 3x + 2 literally stands for a real number; you simply don't know which number until you know which number the variable x stands for. But when we work with that expression, we often think about all of the values that it can take, as x varies, at once. For example, to graph that expression, we don't just draw a point (for a single value) or even really a bunch of points; we draw a line. Geometrically, that line is itself a single complete object in its own right, something more than a point. Algebraically, we can also think of an expression as describing a single complete object in its own right, something more than a number. That something is a function.
Much as we can write the solution set of an inequality as, for example, {x | x < 3}, so we can also write a function as, for example, (x ↦ 3x + 2). However, for historical reasons, that notation is not used in most algebra books. Instead, it is common to give a function a name, just as we might give the value of an expression a name in a word problem, and there is special notation for that. If f stands for our function, then instead of writing f = (x ↦ 3x + 2), we write f(x) = 3x + 2, which means the same thing (if you state it for all values of x). This is convenient notation anyway, because with it we can calculate, for example, that f(5) = 3(5) + 2 = 17. (Remember that, while x here stands for a number, f stands for a function, which is not the same type of thing. So, 3(5) means 3 times 5, and even x(5) would mean x times 5, but f(5) does not work like that.)
Of course, we will also do more of solving equations, graphing expressions, applying algebra in word problems, and everything else that was part of algebra before.
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