The WT21-S1 is an ESP-32-based embedded serial to Ethernet module. The module includes a full TCP/IP stack – etc etc – basically you can use it to program boards normally programmed serially – where that is not practical, by plugging this board into Ethernet via RJ45 and the relevant IO pins and power/ground into the target (ESP8266, Arduino etc).
If you have a use for such a device – it’s cheap! Don’t ask what all those connections are for – basically you would use ground, rx, tx and either 3v3 or 5v for many ESP8266 programming jobs. I tend to simply hold GPIO0 to ground manually on the target while programming (power-up with GPIO grounded, start programming, release) – but I believe that is catered for here. I’ll tell you more once I’ve waded through the docs.
Seeed have an application document if you are interested.