This happens usually when the compiler has seen a forward declaration but no full definition of this type, while the type is being used somewhere. For example:
class A;
class B { A a; };
The second line will cause a compiler error and, depending on the compiler, will report an incomplete type (other compilers give you a different error, but the meaning is the same).
When you however just use a pointer to such a forward declaration no complain will come up, since the size of a pointer to a class is always known. Like this:
class A;
class B {
A *a;
std::shared_ptr<A> aPtr;
};
If you ask what could be wrong in a concrete application or library when this error comes up: that happens usually when a header is included which contains the forward declaration, but full definition hasn’t been found yet. The solution is quite obvious: include also the header that gives you access to the full type. Sometimes you may also simply have no or the wrong namespace used for a type and need to correct that instead.