IV--V--I does refer to positions in the scale, but it's also a bit more specific than that.
IV--V--I in C is F--G--C, and you can see those pitches in the lowest voice.
But these Roman numerals also indicate specific chords whose roots are the given scale degrees. Thus IV indicates a IV chord (F--A--C), V a V chord (G--B--D), and I a I chord (C--E--G).
If you look at the vertical "slices" of each chord, you'll see that the first chord is in fact F--A--C, the second G--B--D, and the last C--E--G. So this is a textbook IV--V--I progression!
Some further clarifications, helpfully suggested in the comments:
- You mention that
F G C should "go higher then lower." In this kind of music, however, we have the notation of octave equivalence, which means that a C is equivalent to any other C. So the bass in this example could go F up to G and then down to C, or it could do F up to G and up again to C. Both are equivalent.
- And as Todd says, although triads comprise three pitches, pitches within the triad can be doubled. In the first chord, for instance, from bottom to top, we have
F A F C. The second F just repeats what we already have, so we really just conceptualize this as
F A C, and thus it's a clear triad. You'll probably learn soon what "inversions" are, which is when the bottom note of this chord can be either A or C, and not just F!