This code is part of Lecture 06 Data Types: Character Here is what we read on Arduino Official page A data type used to store a character value. Character literals are written in single quotes, like this: 'A' (for multiple characters - strings - use double quotes: "ABC"). Characters are stored as numbers however. You can see the specific encoding in the ASCII chart. This means that it is possible to do arithmetic on characters, in which the ASCII value of the character is used (e.g. 'A' + 1 has the value 66, since the ASCII value of the capital letter A is 65). See Serial.println reference for more on how characters are translated to numbers. The size of the char datatype is at least 8 bits. It’s recommended to only use char for storing characters. For an unsigned, one-byte (8 bit) data type, use the byte data type. Char documentation: https://www.arduino.cc/reference/en/language/variables/data-types/char/