C Language: what does the .mm extension stand for?

The extension .mm is the extension for the C++ compilation unit expected by the Objective-C compiler, while the .m extension is the extension for the C compilation unit expected by the Objective-C compiler.

That's assuming you don't use a compiler flag to override the file extension, as this gentleman has done.

Objective C++, mixes C++ and Objective C. http://en.wikipedia.org/wiki/Objective-C#Objective-C.2B.2B

.mm files are like .cpp files to the Objective-C compiler. They activate the C++ compiler instead of the C compiler.

The following question on performance has additional useful information:

Objective-C, .m / .mm performance difference?