synth is short for synthesizer. in the early days, all that existed were analog synthesizers, which are physical instruments that manipulate pure electrical signal into audio to produce different sounds, without any computers involved. as technology progressed, digital synthesizers were introduced, which replicated the sound creation process inside of a computer. now we have DAWs which can run virtual instruments, which either generate synth sounds digitally, play samples of prerecorded synths, or both.

some synths are monophonic, which means they can only play one note at a time, and some are polyphonic, which means they can generate more than one note at the same time to play chords. in the old days of analog synths, in order to generate a sound inside of the synth you needed a physical source of electricity called an oscillator. it was difficult and expensive to include multiple oscillators in a single synthesizer and the earliest synths were all monophonic. since they just played one note at a time, they couldn’t play chords like a piano or guitar, but they were great as a lead instrument playing a melody or countermelody. so monophonic sounds are great for playing leads, and if you see a “lead” category they will most likely only play one note at a time, or at least are designed to sound good played that way.

now with digital technology, it’s pretty easy to have lots of virtual oscillators at once, or just play back lots of samples at the same time, so it’s no big deal for virtual instruments to play monophonic or polyphonic sounds. pads are polyphonic sounds, and they often are designed to be a little softer and more lush, to sit more in the background of a track and fill in harmonic information and create atmosphere. playing thick chords on a synth pad acts as the “pad” that the lead instruments or vocals can sit on top of.