What does implicit declaration mean?

What does implicit declaration of a function mean?

I believe if your compiler is telling you this it is likely that...

a) You haven't included the correct header file for the function that is causing your error.
b) You haven't prototyped your own function, i.e. You're calling it before it's defined.

But I'll caution you I'm not 100% sure on that. May help if you post your code &|| error. (Use [code] tags).