This looks like your precompiled header file.
The precompiled header is shared between all C-dialect files in your project. It's as if all your .c, .cpp, .m and .mm files have an invisible #include directive as the first line. But the Cocoa header files are pure Objective C - trying to include them in a C/C++ source will yield nothing but syntax errors aplenty. Thus the #ifdef.
If your project only contains Objective C files (.m/.mm), which is the typical case, the #ifdef is not really necessary. But Xcode, which generated this header in the first place, protects you all the same.
Even if it's not a PCH file, this #ifdef only makes sense if the file is to be included from both Objective C and plain C/C++. But it does not hurt regardless.