Where to learn about "analog prototype filters"?
"Analog prototype" filters are well-known analog filters that have specific desirable properties. They include (but aren't limited to):


Butterworth filters: maximally-flat passband response
Chebyshev filters: sharper rolloff than Butterworth, equiripple in either the passband or stopband
Elliptic filters: even sharper rolloff, equiripple in passband and stopband
Bessel filters: maximally-flat group delay


These prototypes can be used to design digital filters that have approximately the same characteristics, for instance by using the bilinear transform.