It's for variable argument lists. If you're using C++, then try to avoid ever using it. If you're using C, then try to avoid ever using it, but if you have to then look up VA_ARGS.
Correct is better than fast. Simple is better than complex. Clear is better than cute. Safe is better than insecure.
-- Sutter and Alexandrescu, C++ Coding Standards
Programs must be written for people to read, and only incidentally for machines to execute.
-- Harold Abelson and Gerald Jay Sussman
The cheapest, fastest and most reliable components of a computer system are those that aren't there.
-- Gordon Bell