[Help needed] Significance of ifdef-endif

[rajawatlks]

#ifdef LOCAL
cerr << "Time : " << 1000 * ((double)clock()) / (double)CLOCKS_PER_SEC << “ms\n”; #endif

I have seen the solution of some coders. they use this , can anyone explain me …what is the significance of using this.

[fudu_coder]

C Language: #ifdef Directive

Or maybe even better.
Edit: C/C++ Preprocessor directives | Set 2 - GeeksforGeeks