Linking UNO and ESP8266

No :rofl: it is a Serial signal with a range of somewhere between 0-5v that comes out of the digital TX pin on the UNO and is received by the RX pin on the ESP.

Going the other way… The lower 3.3v TX from the ESP doesn’t bother the UNO’s RX any… but there is also the possibility that because it is just barely above the differentiating voltage the determines HIGH and LOW, that it may not be clear enough. But I have found that with 9600 BAUD it seems just fine.

You can try it without a level shifter, but there are two camps on the concept… one says the ESP will probably be OK, and the other says it will most likely fry the RX pin, if not the whole ESP… and sooner than later.