String += Help

It is indeed a ternary operator:

<condition> ? <value if true> : <value if false>;

A simple example would be:

int valueA = 12;
int valueB = 7;
Serial.println((valueA > 10) ? "Greater than 10" : "Less than 10");
Serial.println((valueB > 10) ? "Greater than 10" : "Less than 10");

This will print:

Greater than 10
Less than 10

P.S: Please use code tags, using the </> button or by using [code][/code] around your code.