This format of numbers and abbreviations (dB/oct = decibels per octave) is often used to refer to the frequency response behavior of a filter. A filter typically has a cutoff or corner frequency it is tuned to. It then reduces (filters) the frequency spectrum of a signal going through it so that its loudness is multiples of 12 decibels weaker for each octave further away you get from the cutoff frequency.
A 12dB/octave filter is often referred to as a “two pole” filter (as each pole of a filter’s design results in 6dB of attenuation). Vintage Arp, Korg, and Oberheim instruments often featured 2-pole filters, often resulting in brighter sounds when compared to those with 4-pole instruments.