The documentation on this is indeed lacking.

If you google define: audio samples, you'll get a fairly clear answer as to what audio samples are, but the simplest explanation is that all digital audio is actually a series of quantized energy levels (samples).

GetOutputData "returns a block of the currently playing source's output data." numSamples would be the number of such samples to return and therefore the size of the returned array. The array returned is an array of float values, representing the quantized value of each sample.

The real question is what "block" is it returning? Is it from the point in the output data where it is currently playing at the moment the function goes through (it reads like this may the case, but doesn't really say) or is it from the beginning of the output data or somewhere else?