What is portamento?
Portamento is a type of Legato playing style which allows for smooth transitions between notes. The easiest way to imagine this is to think of how someone plays a violin, using the full length of the bow with controlled wrist movements to change notes.
To further explain this style of playing, you can think of portamento as the opposite to the short, sharp staccato style of playing, or the pizzicato plucking style on stringed instruments. The style of portamento is more controlled and will let you draw out notes to create sweeping sounds.
Early monophonic synthesizers introduced portamento functions to allow users to smoothly transition or slide from the pitch of one note to another note. Over the years, you might have heard other terms being used for this, common ones being lag, slope generator, slew generator.
Usually controlled by a knob function on the synthesizer, you are able to control the speed of the slide from one pitch to another, as opposed to making an obvious jump from pitch to pitch. The key is to having notes that overlap each other, this gives the portamento room to work with.
Polyphonic synthesizers take advantage of the portamento option by allowing all notes from a chord to arrive at the next pitch at the same time. The distance between each of the notes and the amount they overlap will result in various sounds.
Advanced techniques include applying filter and envelope parameters that are re-triggered with each new note. This is often described as mono mode whereas legato mode will not trigger the filter and/or envelopes at each note.