What does #x inside a C macro mean?

In this context (applied to a parameter reference in a macro definition), the pound sign means to expand this parameter to the literal text of the argument that was passed to the macro.

In this case, if you call PRINT(5) the macro expansion will be printf("5" "%d\n", 5); which will print 5 5; not very useful; however if you call PRINT(5+5) the macro expansion will be printf("5+5" "%d\n", 5+5); which will print 5+5 10, a little less trivial.

This very example is explained in this tutorial on the C preprocessor (which, incidentally, is the first Google hit for c macro pound sign).

"#" can show the name of a variable, it's better to define the macro as this:

#define PRINT(i) printf(#i " = %d\n", i)

and use it like this:

int i = 5;
PRINT(i);

Result shown:

i = 5