Q: What is a Pad?
A "pad" is short for an "Attenuation Pad".
This is a device used to lower the signal level between two other devices.
"attenuator or attenuator pad Electronics. A passive network that reduces the voltage (or power; see usage note under gain) level of a signal with negligible distortion, but with insertion loss. Often a purely resistive network, although any combination of inductors, resistors and capacitors are possible, a pad may also provide impedance matching."
We use them with our Bose Systems when running a pro level +4 dBu signal (e.g. output from a mixer) to the XLR inputs on Channels 1 or 2 on the PS1 Powerstand. We want to do this because the input sensitivity for these inputs is set for microphones. To get a better match for levels between the source and our inputs, we can use a pad.
Values of attenuation are expressed in decibels, dB. dBu/dBV/dBm are ratios/references. dBm is a ratio of power, relative to 1mW. dBu and dBV are ratios of voltage, relative to 0.775V and 1V, respectively. When determining how much of a pad is needed, we want to compare apples-to-apples by converting specs (e.g. http://www.sengpielaudio.com/calculator-db-volt.htm) to a common reference. For the 'mixer line-output to PS1 mic-input' scenario, -20 dB is a good choice because it brings the hot, line-level signal down to a lower voltage range that a mic pre can easily handle.
Pads can also help when connecting the PS1 Line Outs to semi-pro or consumer recording devices. If the PS1 Line Outs are too hot for the recorder's inputs, a pad can be used to prevent overdriving those inputs into (clipping) distortion.
A -20dB pad is also a good choice when connecting the PS1 Line Outs to a house PA, or other off-stage system. This would ensure that the house mixing console's inputs can handle the signal level without being overdriven into distortion, especially if the house console has insufficient built-in pads, or none at all. (Beware of the potential for Ground Loops here.)
Another application (although relatively rare) is if you want to daisy-chain the Line-Out of one PS1 Powerstand to Channel 1 or 2 of another. In that case you probably want to have a -20 dB pad inline, between the Line-Out of the first System and the Channel 1 or 2 XLR input of the second.
You can find separate attenuation pads like this:
This an example, and you will want to look into the details to determine if you need a pad, and the kinds of connections that are appropriate for your input device.