<p>See title: what does it mean for a C++ function to be inline?</p>

The function body is literally inserted inside the caller function. Thus, if you have multiple calls to this function, you get multiple copies of the code. The benefit is you get faster execution.
Usually very short function are inlined, when the copy of the function body would be not much bigger than the usual prologue/epilogue code generated for the normal function call.
You can read more at MSDN article about inline -
http://msdn.microsoft.com/en-us/library/z8y1yy88.aspx