The Filter section is a 24 db/oct lowpass filter that removes high frequencies as its cutoff frequency setting is decreased from max, resulting in a dulling of sound. This a version of the classic transistor "ladder" filter, renowned for its meaty, fat sound.
Keyboard Tracking- This causes the cutoff frequency to increase as ascending notes are played on a keyboard. The idea behind this is, because actual note frequencies rise as higher pitches are played, the Kybd slider adds a rising CV to the cutoff frequency in order to maintain the brightness of notes as higher pitches are played.
Out- Cutoff frequency stays constant across the keyboard - i.e. no tracking.
1/2- Cutoff frequency tracks at half the keyboard CV voltage - i.e. the cutoff frequency rises one octave for each two octaves played on the keyboard.
Full- Cutoff frequency tracks the same as the keyboard CV voltage - i.e. the cutoff frequency rises one octave for each octave played on the keyboard.
Cutoff Freq- Sets the frequency where high-frequency attenuation begins, from 20 Hz up to 24,000 Hz. Though it seems odd to have a cutoff control that goes 4000 Hz above human hearing threshold, remember that all filters have a "slope" to their response, so that extra range guarantees you won't hear any filtering at maximum setting.
Peak Emph- The "emph" is short of for emphasis, commonly referred to as resonance. This emphasizes sound energy at and around the current cutoff frequency by adding feedback from the filter's output back to its input. At lower settings, this can be used to create mild resonances such as those heard in acoustic instruments. MG-1 Plus’ implementation is fully self-oscillating - at extreme settings, it can be used as a sine wave generator, but be careful because high resonance settings can result in loud, screamy, dog-terrifying (and speaker blowing) occurrences.
Contoured Cutoff- The Contoured Cutoff slider applies Contour envelope (i.e. envelope generator) modulation to the cutoff frequency. This allows familiar auto filter sweeps. When raising the Contoured Cutoff control, you'll generally want to lower the Cutoff Freq control, as these work in conjunction with one another.