In my experience it means the linker isn't able to find a function/variable in any of the header files included. Often it means the necessary header file isn't included or the function/variable isn't available in the files included.
another possibility is that the variable or
function is defined in more than one file.
in this case your error message would look
'multiple definition of <identifier>'
'one or more multiply defined symbols'
if you tried to declare a variable in another file
extern int first = 0 ; // definition - multiply defined
extern int second ; // declaration - ok