As Neil said, the SLOT and SIGNAL macros are defined as
#define SLOT(a) "1"#a
#define SIGNAL(a) "2"#a
The #a (with # a stringizing operator) will simply turn whatever is put within the parentheses into a string literal, to create names from the signatures provided to the macros. The "1" and "2" are merely there to distinguish between slots and signals.
This earlier post should provide you some more insight.
If you wonder about the "why?" of all this macro stuff and preprocessing, I would suggest you read up on the "Meta-Object-Compiler" or MOC. And just for fun you could have a look at what MOC does to the code you provide it with. Look through its output and see what it contains. That should be quite informative.
In short, this preprocessing through MOC allows Qt to implement some features (like the signals and slots) which C++ does not provide as standard. (Although there are arguably some implementations of this concept, not related to Qt, which don't require a Meta Object Compiler)
