What does a double hash (##) in a macro mean?

Question

In the below code, what does the ## do?

 #define MAKE_TYPE(myname) \\
 typedef int myname ## Id; \\

Answer 1

icecrime is correct, but something important to point out in the definition is that the tokens need to be valid preprocessing tokens. Examples:

#define CONCAT(a,b) a ## b
CONCAT(ClassyClass, ); // bad,  is not a valid preprocessing token
CONCAT(Symbol, __LINE__); // valid as both are valid tokens