What does $|f (x)|$ mean in the context of Big-O?

Question

In the definition of the 'Big-O Notation', I see the following notation in Discrete Mathematics and Its Applications, 8e, by Kenneth Rosen:

$$|f (x)| ≤ C|g(x)|$$

Now I know that $|\ \ |$ stands for absolute value but does it have the same meaning in the above context as well?