Serial communication
begin, print, println, available, read and printf.
Example
Serial.begin(9600);
Serial.print("x = ");
Serial.println(x);
Serial.println(255, HEX); // FF
if (Serial.available()) { int c = Serial.read(); }The ESP32 also has Serial.printf("%d %.2f\n", a, b); for formatted output. The Uno doesn't.
On the Uno, Serial uses pins 0 (RX) and 1 (TX). Anything connected there disturbs uploads and the monitor.