linker error undefined reference to

I am getting errors saying linker error undefined reference to (function name)
what does this mean? How can I correct it?

it means you didn't write out what that function did, or you didn't link in the correct library.

It means one of these (typically)
- you need to write the function.
- you need to check the spelling - C++ is case sensitive, so 'foo' is different from 'Foo'.
- you need to check the parameters - declaring foo(int a) and defining foo(double a) are different functions.
- you forgot to specify the name of a library.

It could also mean that you forgot to include in the project the file that has the function definition.
Or if the function is a member function of a class, perhaps you forgot the ClassName:: in the definition.