Legato is a playing style that refers to playing a note on the keyboard before you release the previous key you played, as opposed to staccato, in which you release a key before playing the next note. On a synthesizer, legato playing results in new notes not re-triggering the envelope generator, allowing for different articulation between notes depending on whether you play legato or staccato.
Portamento, in the context of synthesis, is essentially Glide between notes - sliding the pitch up or down from one note to the next, rather than discrete steps between two notes, like you’d get on a piano for example. Portamento is more noticeable between large intervals. On synthesizers, portamento is typically a smooth slide, although some synths are equipped with a glissando setting that results in a slide between notes with chromatic stepping, an effect similar to sweeping a piano’s keyboard with your thumb.
On most Moog Synthesizers you can control the time of the glide between two pitches or even the type of glide. A synthesizer like the Subsequent 37 offers a whole range of options. One setting commonly used is LCR (Linear Constant Rate), in which the glide rate will depend on the size of the interval between notes. The larger the interval, the longer the glide time will be. LCR is the most commonly used type of glide. Another common type of glide is called LCT (Linear Constant Time), in which the glide time will be the same, no matter the interval spacing. LCT is commonly used in electronic dance music, particularly in Acid Techno.
Although glide is normally applied to every note you play when engaged, the LEGATO GLIDE function causes glide to occur only when you press a key while still holding a previous key. On Subsequent 37 LEGATO and ON must both be illuminated for LEGATO GLIDE to take effect.