These graphs depict the spectral distributions of an oversampled function and the same function sampled at 1/3 the original rate.  The bandwidth, B, in this example is just small enough that the slower sampling does not cause overlap (aliasing).  Sometimes, a sampled function is resampled at a lower rate by keeping only every Mth sample and discarding the others, commonly called "decimation".  Potential aliasing is prevented by lowpass-filtering the samples before decimation.  The maximum filter bandwidth is tabulated in the bandwidth units used by the common filter design applications.
Own work.  These graphs correspond to the top and bottom graphs of Harris[1], Figure 2.10
Bob K