Compares two Strings, testing whether one comes before or after the other, or whether they're equal. The strings are compared character by character, using the ASCII values of the characters. That means, for example, that 'a' comes before 'b' but after 'A'. Numbers come before letters.
string: a variable of type String
string2: another variable of type String
a negative number: if string comes before string2
0: if string equals string2
a positive number: if string comes after string2