Generally it means they're simulating analog synth components in software; i.e. starting with oscillators producing stuff like triangle or sawtooth waveforms, r

Generally it means they're simulating analog synth components in software; i.e. starting with oscillators producing stuff like triangle or sawtooth waveforms, running it through high pass and low pass filters and an amplitude envelope, throwing in some low-frequency oscillators controlling various parameters, adding some effects like reverb at the end. That way you're doing the same stuff you would in a pure analog synth, and getting similar sounds. In the digital realm there are lots of other ways to make sounds: FM, additive, samples, physical modeling, etc. Pure analog synth hardware had fallen out of favor by the turn of the century but since then it's made a comeback.