Some of the ExpressLRS TX modules include an additional ESP8285 chip, which lets us communicate wirelessly with other ESP8285 enabled devices using a protocol called espnow. We call this chip the "TX-Backpack". The aim of the TX-Backpack is to allow wireless communication between ExpressLRS, and other FPV related devices for command and control, or for querying config.

A prime use case is your video receiver module (or VRX). Currently there aren't many VRX modules that have an ESP8285 built in to allow them to communicate with ExpressLRS, so in most cases you need to add your own. A small ESP based receiver can be "piggybacked" onto your VRX module, which allows ExpressLRS to control the band and channel that your goggles are set to. We call this device the "VRX-Backpack".

The list of supported modules can be found on the wiki: https://github.com/ExpressLRS/Backpack/wiki

There are a few different options for both DIY or compatible off the shelf backpacks... check the wiki for the current list: https://github.com/ExpressLRS/Backpack/wiki