A digital receiver (or AVR), processes input, such as digital audio, received through one or more HDMI ports. It may alter the audio signal using an equalizer or other effects before sending the output to the speakers or another device. In some cases, it is preferable to leave the signal untouched so it can be processed in its original state by another device. The "passthrough" option — typically located in the AVR's audio menu — prevents the input signal from being processed.
HDMI "standby passthrough" is a similar but different option available on some AVRs. It allows digital data to flow through a specific HDMI port even when the device is in standby mode (plugged in, but not turned on).
USB passthrough allows USB peripherals to be daisy-chained through one or more devices. A common example is a keyboard with a USB port. You can plug a mouse into the USB port on the keyboard, which in turn connects to the USB port on the back of the computer. USB hubs provide the same function, often for multiple devices.
USB passthrough passes digital data through a device, but it does not always provide USB power. If a USB device requires electrical power to operate (or charge), the passthrough device must supply the necessary wattage. Keyboards with powered USB passthrough typically use two USB cables. USB hubs that provide USB power may include an AC adaptor.
In networking, passthrough may refer to any device that relays data unaltered to another device on the network. However, it may also refer specifically to a modem or router that passes the IP address to a connected device.
IP passthrough is a setting that turns off the device's routing features and lets the data pass through to the next connected device. It is similar to "bridge mode," but also removes any firewall features or other data processing. IP passthrough eliminates the need for network address translation (NAT) and assigns the public IP address to the connected device.