It's a function declaration that is used either in a header file to provide the signature for external functions (i.e. not in this file, but in another or in a library) so it can be called from your code; or to give the signature for a function before the actual definition of the function so it can be used by other functions (this is known as a forward declaration).
For example, if you have two functions that call each other:
void A()
{
B();
}
void B()
{
A();
}
Then you need at least one forward declaration (of B) or it's signature is not known when it is first called.