The Discriminator output from a scanner is the raw, unfiltered signal that a scanner produces before it is sent to the audio stage for output through the speaker -- also called the "baseband audio." This discriminator source is required for applications such as Trunker, T4Win, UniTrunker, and DSD. Most scanners must have a simple modification made to them to feed the discriminator output to Data Slicers, or in the case of UniTrunker, a soundcard.
Note that for providing a signal for ACARS or MPT-1327 decoding, a connection from an earphone or speaker output is all that's required. This is because these modes run at a much lower speed, and the discriminator is not involved in AM reception (for ACARS decoding).
Also see the Scandata faq q2 article for a list of older scanners' tap points. This is from an original article written by the original scanner hacker and guru the late Bill Cheek.
Why use a resistor and/or capacitor ?
See #6 through 8 of this URL: Discriminator.nl FAQ
Wikipedia Page showing Circuit and calculations for Discriminator Tap.
These pages or links contain more information on discriminator taps for many brands and models of scanners, including older models