The hold switch, within the context of iPods, are mechanical switches that, when toggled, disables input to the iPod, whether from the scroll wheel, side buttons, touch wheel, Click Wheel, or the center button.
When the hold switch is on, you can see the orange color on it. The iPod 5.5 Gen U2 Edition is an exception to this, as this iPod's hold switch has red instead of orange.
The hold switch is present on all full-size iPods, all models of iPod mini, and the first through fifth generations of the iPod nano.
Replacement hold switches form a part of the headphone assembly of iPods, and are readily available for the iPod video and iPod classic.
The hold switch shares the same design as the current silent switch on iPhones. It also looks similar to the lock button on iPod touch, first through fourth generations.
It is to note that all full-size iPod models' clickers are located on the hold switch assembly.
Resources
About the Hold Switch (Dummies.com): https://www.dummies.com/consumer-electronics/mp3-players/ipod/your-ipods-hold-switch/
About the iPhone Silent Switch (similar in design to the hold switch, but has a different function): https://support.apple.com/en-us/HT208353
Images
Image of hold switch's location for an iPod Classic
Image of the iPod Video's hold switch (from a real iPod)
Image of the iPod Video's (5 & 5.5) hold switch location.
Location of the hold switch for an iPod Nano 3rd Generation.
