Technically, nodal points are stationary spots on an object that is in one of it's 'normal modes' of vibration. The handiest examples of this are seen in the strings of your guitar. When you pluck the string it is vibrating everywhere except for the bridge saddle and nut, which are 'nodes' for all of the vibration modes of the string. If you touch the string in the exact center, over the 12th fret, you supress a whole family of the modes; the 'fundamental', and all the ones that are at odd-number multiples of the fundamental frequency. This happens because that center point is an 'antinode' for those modes: it has to be moving at those pitches, but it is a 'node' for all of the even-order partials of the string. That's why the string sounds an octave higher. If you touch the string in a different place, you supress a different set of modes, and get a different sound.
A string has nodal _points_ because it's essentially a one-dimensional object: in terms of vibration all you need to think about most of the time is the length, with the thickness being very small in relation to that. If there are vibration modes _across_ the string, they're too high up in pitch to worry us!
A two dimensional object, like a drum head, has more complicated modes, which involve moving areas seperated by lines. One outcome of the extra complication is that the pitches of the different modes are not as simply related as those of the string. That's why tapping on a drum usually makes a 'noise' rather than a more musical 'tone'. Guitar tops are even more complicated, due to the nature of wood, and the addition of bracing, so the modal maps tend to be more complicated still, and more difficult to predict.
One thing that happens on these complicated objects, though, is that node lines tend to run either along or between areas of high mass or high stiffness. Those peaks on the braces tend to impose nodes, or 'antinodes' (areas of maximum amplitude) where they occur, in part because of their mass, but more because of their stiffness. It's like your finger touching the string at the 12th fret, but a little more complicated because the brace is also part of the top that's vibrating.
So, I would not exactly call those 'node points', but they are 'points that are likely to be nodes' for a number of different modes of vibration of the top. How all of this effects the sound is the $64,000 question, and varies from one guitar to the next (you didn't really expect this to be simple, did you?).
What can be said in general is that different brace profiling schemes tend to enhance or supress different modes of vibration of the top, and alter the pitches of them. These modes are the major 'formants' of the tone, that give each instrument it's character. Guitars with 'scalloped' bracing tend to sound one way, and 'tapered' braced guitars are different. It all gets complicated fast, because everybody does things a little differently, and we're working with wood, that varies all over the place in it's mechanical and acoustic properties.
So there you are, ask a simple question......
