The Pico has two UARTs for serial communication.
UART0 is dedicated to the MicroPython REPL (Read-Evaluate-Print_loop), but you can use UART1 in your programs.
Like all the Pico devices, the UARTs should only be used with 3V3 signals.
I used an FTDI TTL-232R-3V3 cable with some jump wires to connect my Linux computer to the Pico.
This code (in src/pico/experiments/uart-test.py) will display 'Hello UART' and then print a line for every character you type on the host.
from machine import UART
ser = UART(1, 115200)
ser.write('Hello UART!\r\n')
while True:
ch = ser.read(1)
reply = 'I saw %s \r\n' % ch
ser.write(reply)
print(reply)