What do "peak points" of wavelength correspond to / stand for?
The peak points of chlorophyll and their correspondence to the action spectrum indicate that the Chlorophylls play a major role in photosynthesis, it is also clear that chlorophyll isn't the only chemical involved in light harvesting and that other antenna pigments are utilized.
As mentioned in Chris' answer chlorophylls (plural) absorb and reflect light to a various degree at different frequencies, as explained on webpage in the link that was provided (in the section photosynthesis).
From Wikipedia's Chlorophyll webpage, "Measurement of chlorophyll content" section:
"In 90% acetone-water, the peak absorption wavelengths of chlorophyll a are 430 nm and 664 nm; peaks for chlorophyll b are 460 nm and 647 nm; chlorophyll c has a few isomers, peaks for c$_1$ are 442 nm and 630 nm; peaks for chlorophyll c$_2$ are 444 nm and 630 nm; variety c$_3$ is recently discovered; peaks for chlorophyll d are 401 nm, 455 nm and 696 nm;" chlorophyll f (also a newer discovery than information offered on Wikipedia) absorbs at 720nm - making it the most red-shifted chlorophyll to date.
Notice that chlorophyll A's Soret peak provides the greatest rate of growth. Blue light also affects phototropism more than red. Different wavelengths affect the plant differently, some cause growth while other wavelengths regulate bending, even seed development.
Here's an image from cell.us's webpage: "Action Spectrum Green Plants":
Note that plants reflect infrared light beyond 700nm, that is why they appear white (instead of black) in the right side infrared photograph:
Click to zoom
How does light peaking in red spectrum (say around 630nm) differ to (or affect plants) compared to light in red spectrum peaking at 680nm (which I gather is Chlorophyll A peak-ish)?
Chlorophyll b simply expands the range of usable light available to the plant.
When same amount of radiant flux is projected to leaves on different wavelengths, one far from a chlorophyll peak the other near, which one gives more energy to plant and why?
Light frequencies closest to one of the peaks has the most affect on growth, as you can see from the action spectrum blue light is more energetic than red and even mid-band (green) light has a small but significant effect on growth. Because green light is not absorbed some is reflected and some passes through the upper leaves to the lower shaded leaves.
Plants use light inefficiently because it isn't a limiting factor in its growth, instead CO$_2$ is.