I have no idea what that is.
Its a little microcontroller similar to an Arduino or Pico with builtin wifi. ESP32 is just one popular example of similar chips. About 10 bucks. It can be a server or a client. But it has its own GPIO like the vex brain has ports.
You can do stuff like press a webpage button that controls your vex robot, say opening a claw. The ESP32 has its own code (which you write) that can (among other things) turn a signal from the wifi into a digital high on one of the pins. That pin is plugged into one of the vex brain’s 3-wire ports you’ve configured as a digital input. Finally, the code on your vex brain takes that signal and uses it to activate something like an if, while, etc.
I also like it because whereas the VEXnet protocol is hidden, an external connection like that uses normal wifi. So its a bit more educational if you are teaching network stuff.