Nope, not the switch on your microphone preamp.
Pads are generally soft sustained notes in the background. Most synths have a preset category for this. They often sound impressive (or silly) when playing big two-handed chords.
They’re most commonly used to support the current chord and fill out the harmonic (and spectral) texture. For example, if your bassline is sitting mainly on the tonic (root note of the current chord) and your melody is mainly hovering around the dominant (fifth above the root note), a pad can add richness by bringing in the mediant (3rd) and even the subtonic (7th). You can also use a pad to bring in other other degrees (such as the 4th, 6th, 9th or even 11th) in a subtle way. While this can add a lot of harmonic richness, you have to be careful not to make it too obvious – otherwise the harmonic structure will be too dense and messy. It might be useful for an occasional effect, however, so don’t rule it out entirely.
Pads are also useful in filling out the spectral texture of a mix. By having a relatively broad spectral bandwidth, pads can fill the sonic gaps left between the foreground instruments. Like the harmonic structure, this can add richness and depth to the sound. Pads often sit naturally in the background because they’re quite diffuse, soft and slow-moving. They can be particularly useful for adding stereo width to an other-wise narrow mix. They’re prime candidates for subtle stereo-enhancing processes because – as a background part – the mix won’t suffer too much if the pads disappear a bit when collapsed to mono. If anything, it could be beneficial because it’ll make the mix more focussed in mono, reducing the effect of having all the instruments stepping on top of each other.
Pads aren’t just for electronic music! In rock and pop, distorted guitars and backing vocals often take the role of a pad – providing a diffuse background texture to support foreground instruments and fill out the sound. Orchestral strings (or string-like textures) are often called upon to fill this role too.